cpl_virtualmem.cpp

/**********************************************************************
 *
 * Name:     cpl_virtualmem.cpp
 * Project:  CPL - Common Portability Library
 * Purpose:  Virtual memory
 * Author:   Even Rouault, <even dot rouault at spatialys.com>
 *
 **********************************************************************
 * Copyright (c) 2014, Even Rouault <even dot rouault at spatialys.com>
 *
 * SPDX-License-Identifier: MIT
 ****************************************************************************/

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif

// to have off_t on 64bit possibly
#ifndef _FILE_OFFSET_BITS
#define _FILE_OFFSET_BITS 64
#endif

#include "cpl_virtualmem.h"

#include <cassert>
// TODO(schwehr): Should ucontext.h be included?
// #include <ucontext.h>

#include "cpl_atomic_ops.h"
#include "cpl_config.h"
#include "cpl_conv.h"
#include "cpl_error.h"
#include "cpl_multiproc.h"

#ifdef NDEBUG
// Non NDEBUG: Ignore the result.
#define IGNORE_OR_ASSERT_IN_DEBUG(expr) CPL_IGNORE_RET_VAL((expr))
#else
// Debug: Assert.
#define IGNORE_OR_ASSERT_IN_DEBUG(expr) assert((expr))
#endif

#if defined(__linux) && defined(CPL_MULTIPROC_PTHREAD)
#ifndef HAVE_5ARGS_MREMAP
// FIXME? gcore/virtualmem.py tests fail/crash when HAVE_5ARGS_MREMAP
// is not defined.
#warning "HAVE_5ARGS_MREMAP not found. Disabling HAVE_VIRTUAL_MEM_VMA"
#else
#define HAVE_VIRTUAL_MEM_VMA
#endif
#endif

#if defined(HAVE_MMAP) || defined(HAVE_VIRTUAL_MEM_VMA)
#include <unistd.h>    // read, write, close, pipe, sysconf
#include <sys/mman.h>  // mmap, munmap, mremap
#endif

typedef enum
{
    VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED,
    VIRTUAL_MEM_TYPE_VMA
} CPLVirtualMemType;

struct CPLVirtualMem
{
    CPLVirtualMemType eType;

    struct CPLVirtualMem *pVMemBase;
    int nRefCount;

    CPLVirtualMemAccessMode eAccessMode;

    size_t nPageSize;
    // Aligned on nPageSize.
    void *pData;
    // Returned by mmap(), potentially lower than pData.
    void *pDataToFree;
    // Requested size (unrounded).
    size_t nSize;

    bool bSingleThreadUsage;

    void *pCbkUserData;
    CPLVirtualMemFreeUserData pfnFreeUserData;
};

#ifdef HAVE_VIRTUAL_MEM_VMA

#include <sys/select.h>  // select
#include <sys/stat.h>    // open()
#include <sys/types.h>   // open()
#include <errno.h>
#include <fcntl.h>   // open()
#include <signal.h>  // sigaction
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

#ifndef HAVE_5ARGS_MREMAP
#include "cpl_atomic_ops.h"
#endif

/* Linux specific (i.e. non POSIX compliant) features used:
   - returning from a SIGSEGV handler is clearly a POSIX violation, but in
     practice most POSIX systems should be happy.
   - mremap() with 5 args is Linux specific. It is used when the user
     callback is invited to fill a page, we currently mmap() a
     writable page, let it filled it, and afterwards mremap() that
     temporary page onto the location where the fault occurred.
     If we have no mremap(), the workaround is to pause other threads that
     consume the current view while we are updating the faulted page, otherwise
     a non-paused thread could access a page that is in the middle of being
     filled... The way we pause those threads is quite original : we send them
     a SIGUSR1 and wait that they are stuck in the temporary SIGUSR1 handler...
   - MAP_ANONYMOUS isn't documented in POSIX, but very commonly found
     (sometimes called MAP_ANON)
   - dealing with the limitation of number of memory mapping regions,
     and the 65536 limit.
   - other things I've not identified
*/

#define ALIGN_DOWN(p, pagesize)                                                \
    reinterpret_cast<void *>((reinterpret_cast<GUIntptr_t>(p)) / (pagesize) *  \
                             (pagesize))
#define ALIGN_UP(p, pagesize)                                                  \
    reinterpret_cast<void *>(                                                  \
        (reinterpret_cast<GUIntptr_t>(p) + (pagesize)-1) / (pagesize) *        \
        (pagesize))

#define DEFAULT_PAGE_SIZE (256 * 256)
#define MAXIMUM_PAGE_SIZE (32 * 1024 * 1024)

// Linux Kernel limit.
#define MAXIMUM_COUNT_OF_MAPPINGS 65536

#define BYEBYE_ADDR (reinterpret_cast<void *>(~static_cast<size_t>(0)))

#define MAPPING_FOUND "yeah"
#define MAPPING_NOT_FOUND "doh!"

#define SET_BIT(ar, bitnumber) ar[(bitnumber) / 8] |= 1 << ((bitnumber) % 8)
#define UNSET_BIT(ar, bitnumber)                                               \
    ar[(bitnumber) / 8] &= ~(1 << ((bitnumber) % 8))
#define TEST_BIT(ar, bitnumber) (ar[(bitnumber) / 8] & (1 << ((bitnumber) % 8)))

typedef enum
{
    OP_LOAD,
    OP_STORE,
    OP_MOVS_RSI_RDI,
    OP_UNKNOWN
} OpType;

typedef struct
{
    CPLVirtualMem sBase;

    GByte *pabitMappedPages;
    GByte *pabitRWMappedPages;

    int nCacheMaxSizeInPages;  // Maximum size of page array.
    int *panLRUPageIndices;    // Array with indices of cached pages.
    int iLRUStart;             // Index in array where to
                               // write next page index.
    int nLRUSize;              // Current size of the array.

    int iLastPage;  // Last page accessed.
    int nRetry;     // Number of consecutive
                    // retries to that last page.

    CPLVirtualMemCachePageCbk pfnCachePage;      // Called when a page is
                                                 // mapped.
    CPLVirtualMemUnCachePageCbk pfnUnCachePage;  // Called when a (writable)
                                                 // page is unmapped.

#ifndef HAVE_5ARGS_MREMAP
    CPLMutex *hMutexThreadArray;
    int nThreads;
    pthread_t *pahThreads;
#endif
} CPLVirtualMemVMA;

typedef struct
{
    // hVirtualMemManagerMutex protects the 2 following variables.
    CPLVirtualMemVMA **pasVirtualMem;
    int nVirtualMemCount;

    int pipefd_to_thread[2];
    int pipefd_from_thread[2];
    int pipefd_wait_thread[2];
    CPLJoinableThread *hHelperThread;

    // Using sigaction without testing HAVE_SIGACTION since we are in a Linux
    // specific code path
    struct sigaction oldact;
} CPLVirtualMemManager;

typedef struct
{
    void *pFaultAddr;
    OpType opType;
    pthread_t hRequesterThread;
} CPLVirtualMemMsgToWorkerThread;

// TODO: Singletons.
static CPLVirtualMemManager *pVirtualMemManager = nullptr;
static CPLMutex *hVirtualMemManagerMutex = nullptr;

static bool CPLVirtualMemManagerInit();

#ifdef DEBUG_VIRTUALMEM

/************************************************************************/
/*                           fprintfstderr()                            */
/************************************************************************/

// This function may be called from signal handlers where most functions
// from the C library are unsafe to be called. fprintf() is clearly one
// of those functions (see
// http://stackoverflow.com/questions/4554129/linux-glibc-can-i-use-fprintf-in-signal-handler)
// vsnprintf() is *probably* safer with respect to that (but there is no
// guarantee though).
// write() is async-signal-safe.
static void fprintfstderr(const char *fmt, ...)
{
    char buffer[80] = {};
    va_list ap;
    va_start(ap, fmt);
    vsnprintf(buffer, sizeof(buffer), fmt, ap);
    va_end(ap);
    int offset = 0;
    while (true)
    {
        const size_t nSizeToWrite = strlen(buffer + offset);
        int ret = static_cast<int>(write(2, buffer + offset, nSizeToWrite));
        if (ret < 0 && errno == EINTR)
        {
        }
        else
        {
            if (ret == static_cast<int>(nSizeToWrite))
                break;
            offset += ret;
        }
    }
}

#endif

/************************************************************************/
/*              CPLVirtualMemManagerRegisterVirtualMem()                */
/************************************************************************/

static bool CPLVirtualMemManagerRegisterVirtualMem(CPLVirtualMemVMA *ctxt)
{
    if (!CPLVirtualMemManagerInit())
        return false;

    bool bSuccess = true;
    IGNORE_OR_ASSERT_IN_DEBUG(ctxt);
    CPLAcquireMutex(hVirtualMemManagerMutex, 1000.0);
    CPLVirtualMemVMA **pasVirtualMemNew = static_cast<CPLVirtualMemVMA **>(
        VSI_REALLOC_VERBOSE(pVirtualMemManager->pasVirtualMem,
                            sizeof(CPLVirtualMemVMA *) *
                                (pVirtualMemManager->nVirtualMemCount + 1)));
    if (pasVirtualMemNew == nullptr)
    {
        bSuccess = false;
    }
    else
    {
        pVirtualMemManager->pasVirtualMem = pasVirtualMemNew;
        pVirtualMemManager
            ->pasVirtualMem[pVirtualMemManager->nVirtualMemCount] = ctxt;
        pVirtualMemManager->nVirtualMemCount++;
    }
    CPLReleaseMutex(hVirtualMemManagerMutex);
    return bSuccess;
}

/************************************************************************/
/*               CPLVirtualMemManagerUnregisterVirtualMem()             */
/************************************************************************/

static void CPLVirtualMemManagerUnregisterVirtualMem(CPLVirtualMemVMA *ctxt)
{
    CPLAcquireMutex(hVirtualMemManagerMutex, 1000.0);
    for (int i = 0; i < pVirtualMemManager->nVirtualMemCount; i++)
    {
        if (pVirtualMemManager->pasVirtualMem[i] == ctxt)
        {
            if (i < pVirtualMemManager->nVirtualMemCount - 1)
            {
                memmove(pVirtualMemManager->pasVirtualMem + i,
                        pVirtualMemManager->pasVirtualMem + i + 1,
                        sizeof(CPLVirtualMem *) *
                            (pVirtualMemManager->nVirtualMemCount - i - 1));
            }
            pVirtualMemManager->nVirtualMemCount--;
            break;
        }
    }
    CPLReleaseMutex(hVirtualMemManagerMutex);
}

/************************************************************************/
/*                           CPLVirtualMemNew()                         */
/************************************************************************/

static void CPLVirtualMemFreeFileMemoryMapped(CPLVirtualMemVMA *ctxt);

CPLVirtualMem *CPLVirtualMemNew(size_t nSize, size_t nCacheSize,
                                size_t nPageSizeHint, int bSingleThreadUsage,
                                CPLVirtualMemAccessMode eAccessMode,
                                CPLVirtualMemCachePageCbk pfnCachePage,
                                CPLVirtualMemUnCachePageCbk pfnUnCachePage,
                                CPLVirtualMemFreeUserData pfnFreeUserData,
                                void *pCbkUserData)
{
    size_t nMinPageSize = CPLGetPageSize();
    size_t nPageSize = DEFAULT_PAGE_SIZE;

    IGNORE_OR_ASSERT_IN_DEBUG(nSize > 0);
    IGNORE_OR_ASSERT_IN_DEBUG(pfnCachePage != nullptr);

    if (nPageSizeHint >= nMinPageSize && nPageSizeHint <= MAXIMUM_PAGE_SIZE)
    {
        if ((nPageSizeHint % nMinPageSize) == 0)
            nPageSize = nPageSizeHint;
        else
        {
            int nbits = 0;
            nPageSize = static_cast<size_t>(nPageSizeHint);
            do
            {
                nPageSize >>= 1;
                nbits++;
            } while (nPageSize > 0);
            nPageSize = static_cast<size_t>(1) << (nbits - 1);
            if (nPageSize < static_cast<size_t>(nPageSizeHint))
                nPageSize <<= 1;
        }
    }

    if ((nPageSize % nMinPageSize) != 0)
        nPageSize = nMinPageSize;

    if (nCacheSize > nSize)
        nCacheSize = nSize;
    else if (nCacheSize == 0)
        nCacheSize = 1;

    int nMappings = 0;

    // Linux specific:
    // Count the number of existing memory mappings.
    FILE *f = fopen("/proc/self/maps", "rb");
    if (f != nullptr)
    {
        char buffer[80] = {};
        while (fgets(buffer, sizeof(buffer), f) != nullptr)
            nMappings++;
        fclose(f);
    }

    size_t nCacheMaxSizeInPages = 0;
    while (true)
    {
        // /proc/self/maps must not have more than 65K lines.
        nCacheMaxSizeInPages = (nCacheSize + 2 * nPageSize - 1) / nPageSize;
        if (nCacheMaxSizeInPages >
            static_cast<size_t>((MAXIMUM_COUNT_OF_MAPPINGS * 9 / 10) -
                                nMappings))
            nPageSize <<= 1;
        else
            break;
    }
    size_t nRoundedMappingSize =
        ((nSize + 2 * nPageSize - 1) / nPageSize) * nPageSize;
    void *pData = mmap(nullptr, nRoundedMappingSize, PROT_NONE,
                       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (pData == MAP_FAILED)
    {
        perror("mmap");
        return nullptr;
    }
    CPLVirtualMemVMA *ctxt = static_cast<CPLVirtualMemVMA *>(
        VSI_CALLOC_VERBOSE(1, sizeof(CPLVirtualMemVMA)));
    if (ctxt == nullptr)
    {
        munmap(pData, nRoundedMappingSize);
        return nullptr;
    }
    ctxt->sBase.nRefCount = 1;
    ctxt->sBase.eType = VIRTUAL_MEM_TYPE_VMA;
    ctxt->sBase.eAccessMode = eAccessMode;
    ctxt->sBase.pDataToFree = pData;
    ctxt->sBase.pData = ALIGN_UP(pData, nPageSize);
    ctxt->sBase.nPageSize = nPageSize;
    ctxt->sBase.nSize = nSize;
    ctxt->sBase.bSingleThreadUsage = CPL_TO_BOOL(bSingleThreadUsage);
    ctxt->sBase.pfnFreeUserData = pfnFreeUserData;
    ctxt->sBase.pCbkUserData = pCbkUserData;

    ctxt->pabitMappedPages = static_cast<GByte *>(
        VSI_CALLOC_VERBOSE(1, (nRoundedMappingSize / nPageSize + 7) / 8));
    if (ctxt->pabitMappedPages == nullptr)
    {
        CPLVirtualMemFreeFileMemoryMapped(ctxt);
        CPLFree(ctxt);
        return nullptr;
    }
    ctxt->pabitRWMappedPages = static_cast<GByte *>(
        VSI_CALLOC_VERBOSE(1, (nRoundedMappingSize / nPageSize + 7) / 8));
    if (ctxt->pabitRWMappedPages == nullptr)
    {
        CPLVirtualMemFreeFileMemoryMapped(ctxt);
        CPLFree(ctxt);
        return nullptr;
    }
    // Need at least 2 pages in case for a rep movs instruction
    // that operate in the view.
    ctxt->nCacheMaxSizeInPages = static_cast<int>(nCacheMaxSizeInPages);
    ctxt->panLRUPageIndices = static_cast<int *>(
        VSI_MALLOC_VERBOSE(ctxt->nCacheMaxSizeInPages * sizeof(int)));
    if (ctxt->panLRUPageIndices == nullptr)
    {
        CPLVirtualMemFreeFileMemoryMapped(ctxt);
        CPLFree(ctxt);
        return nullptr;
    }
    ctxt->iLRUStart = 0;
    ctxt->nLRUSize = 0;
    ctxt->iLastPage = -1;
    ctxt->nRetry = 0;
    ctxt->pfnCachePage = pfnCachePage;
    ctxt->pfnUnCachePage = pfnUnCachePage;

#ifndef HAVE_5ARGS_MREMAP
    if (!ctxt->sBase.bSingleThreadUsage)
    {
        ctxt->hMutexThreadArray = CPLCreateMutex();
        IGNORE_OR_ASSERT_IN_DEBUG(ctxt->hMutexThreadArray != nullptr);
        CPLReleaseMutex(ctxt->hMutexThreadArray);
        ctxt->nThreads = 0;
        ctxt->pahThreads = nullptr;
    }
#endif

    if (!CPLVirtualMemManagerRegisterVirtualMem(ctxt))
    {
        CPLVirtualMemFreeFileMemoryMapped(ctxt);
        CPLFree(ctxt);
        return nullptr;
    }

    return reinterpret_cast<CPLVirtualMem *>(ctxt);
}

/************************************************************************/
/*                  CPLVirtualMemFreeFileMemoryMapped()                 */
/************************************************************************/

static void CPLVirtualMemFreeFileMemoryMapped(CPLVirtualMemVMA *ctxt)
{
    CPLVirtualMemManagerUnregisterVirtualMem(ctxt);

    size_t nRoundedMappingSize =
        ((ctxt->sBase.nSize + 2 * ctxt->sBase.nPageSize - 1) /
         ctxt->sBase.nPageSize) *
        ctxt->sBase.nPageSize;
    if (ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE &&
        ctxt->pabitRWMappedPages != nullptr && ctxt->pfnUnCachePage != nullptr)
    {
        for (size_t i = 0; i < nRoundedMappingSize / ctxt->sBase.nPageSize; i++)
        {
            if (TEST_BIT(ctxt->pabitRWMappedPages, i))
            {
                void *addr = static_cast<char *>(ctxt->sBase.pData) +
                             i * ctxt->sBase.nPageSize;
                ctxt->pfnUnCachePage(reinterpret_cast<CPLVirtualMem *>(ctxt),
                                     i * ctxt->sBase.nPageSize, addr,
                                     ctxt->sBase.nPageSize,
                                     ctxt->sBase.pCbkUserData);
            }
        }
    }
    int nRet = munmap(ctxt->sBase.pDataToFree, nRoundedMappingSize);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
    CPLFree(ctxt->pabitMappedPages);
    CPLFree(ctxt->pabitRWMappedPages);
    CPLFree(ctxt->panLRUPageIndices);
#ifndef HAVE_5ARGS_MREMAP
    if (!ctxt->sBase.bSingleThreadUsage)
    {
        CPLFree(ctxt->pahThreads);
        CPLDestroyMutex(ctxt->hMutexThreadArray);
    }
#endif
}

#ifndef HAVE_5ARGS_MREMAP

static volatile int nCountThreadsInSigUSR1 = 0;
static volatile int nWaitHelperThread = 0;

/************************************************************************/
/*                   CPLVirtualMemSIGUSR1Handler()                      */
/************************************************************************/

static void CPLVirtualMemSIGUSR1Handler(int /* signum_unused */,
                                        siginfo_t * /* the_info_unused */,
                                        void * /* the_ctxt_unused */)
{
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
    fprintfstderr("entering CPLVirtualMemSIGUSR1Handler %X\n", pthread_self());
#endif
    // Rouault guesses this is only POSIX correct if it is implemented by an
    // intrinsic.
    CPLAtomicInc(&nCountThreadsInSigUSR1);
    while (nWaitHelperThread)
        // Not explicitly indicated as signal-async-safe, but hopefully ok.
        usleep(1);
    CPLAtomicDec(&nCountThreadsInSigUSR1);
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
    fprintfstderr("leaving CPLVirtualMemSIGUSR1Handler %X\n", pthread_self());
#endif
}
#endif

/************************************************************************/
/*                      CPLVirtualMemDeclareThread()                    */
/************************************************************************/

void CPLVirtualMemDeclareThread(CPLVirtualMem *ctxt)
{
    if (ctxt->eType == VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED)
        return;
#ifndef HAVE_5ARGS_MREMAP
    CPLVirtualMemVMA *ctxtVMA = reinterpret_cast<CPLVirtualMemVMA *>(ctxt);
    IGNORE_OR_ASSERT_IN_DEBUG(!ctxt->bSingleThreadUsage);
    CPLAcquireMutex(ctxtVMA->hMutexThreadArray, 1000.0);
    ctxtVMA->pahThreads = static_cast<pthread_t *>(CPLRealloc(
        ctxtVMA->pahThreads, (ctxtVMA->nThreads + 1) * sizeof(pthread_t)));
    ctxtVMA->pahThreads[ctxtVMA->nThreads] = pthread_self();
    ctxtVMA->nThreads++;

    CPLReleaseMutex(ctxtVMA->hMutexThreadArray);
#endif
}

/************************************************************************/
/*                     CPLVirtualMemUnDeclareThread()                   */
/************************************************************************/

void CPLVirtualMemUnDeclareThread(CPLVirtualMem *ctxt)
{
    if (ctxt->eType == VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED)
        return;
#ifndef HAVE_5ARGS_MREMAP
    CPLVirtualMemVMA *ctxtVMA = reinterpret_cast<CPLVirtualMemVMA *>(ctxt);
    pthread_t self = pthread_self();
    IGNORE_OR_ASSERT_IN_DEBUG(!ctxt->bSingleThreadUsage);
    CPLAcquireMutex(ctxtVMA->hMutexThreadArray, 1000.0);
    for (int i = 0; i < ctxtVMA->nThreads; i++)
    {
        if (ctxtVMA->pahThreads[i] == self)
        {
            if (i < ctxtVMA->nThreads - 1)
                memmove(ctxtVMA->pahThreads + i + 1, ctxtVMA->pahThreads + i,
                        (ctxtVMA->nThreads - 1 - i) * sizeof(pthread_t));
            ctxtVMA->nThreads--;
            break;
        }
    }

    CPLReleaseMutex(ctxtVMA->hMutexThreadArray);
#endif
}

/************************************************************************/
/*                     CPLVirtualMemGetPageToFill()                     */
/************************************************************************/

// Must be paired with CPLVirtualMemAddPage.
static void *CPLVirtualMemGetPageToFill(CPLVirtualMemVMA *ctxt,
                                        void *start_page_addr)
{
    void *pPageToFill = nullptr;

    if (ctxt->sBase.bSingleThreadUsage)
    {
        pPageToFill = start_page_addr;
        const int nRet = mprotect(pPageToFill, ctxt->sBase.nPageSize,
                                  PROT_READ | PROT_WRITE);
        IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
    }
    else
    {
#ifndef HAVE_5ARGS_MREMAP
        CPLAcquireMutex(ctxt->hMutexThreadArray, 1000.0);
        if (ctxt->nThreads == 1)
        {
            pPageToFill = start_page_addr;
            const int nRet = mprotect(pPageToFill, ctxt->sBase.nPageSize,
                                      PROT_READ | PROT_WRITE);
            IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
        }
        else
#endif
        {
            // Allocate a temporary writable page that the user
            // callback can fill.
            pPageToFill =
                mmap(nullptr, ctxt->sBase.nPageSize, PROT_READ | PROT_WRITE,
                     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
            IGNORE_OR_ASSERT_IN_DEBUG(pPageToFill != MAP_FAILED);
        }
    }
    return pPageToFill;
}

/************************************************************************/
/*                        CPLVirtualMemAddPage()                        */
/************************************************************************/

static void CPLVirtualMemAddPage(CPLVirtualMemVMA *ctxt, void *target_addr,
                                 void *pPageToFill, OpType opType,
                                 pthread_t hRequesterThread)
{
    const int iPage =
        static_cast<int>((static_cast<char *>(target_addr) -
                          static_cast<char *>(ctxt->sBase.pData)) /
                         ctxt->sBase.nPageSize);
    if (ctxt->nLRUSize == ctxt->nCacheMaxSizeInPages)
    {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
        fprintfstderr("uncaching page %d\n", iPage);
#endif
        int nOldPage = ctxt->panLRUPageIndices[ctxt->iLRUStart];
        void *addr = static_cast<char *>(ctxt->sBase.pData) +
                     nOldPage * ctxt->sBase.nPageSize;
        if (ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE &&
            ctxt->pfnUnCachePage != nullptr &&
            TEST_BIT(ctxt->pabitRWMappedPages, nOldPage))
        {
            size_t nToBeEvicted = ctxt->sBase.nPageSize;
            if (static_cast<char *>(addr) + nToBeEvicted >=
                static_cast<char *>(ctxt->sBase.pData) + ctxt->sBase.nSize)
                nToBeEvicted = static_cast<char *>(ctxt->sBase.pData) +
                               ctxt->sBase.nSize - static_cast<char *>(addr);

            ctxt->pfnUnCachePage(reinterpret_cast<CPLVirtualMem *>(ctxt),
                                 nOldPage * ctxt->sBase.nPageSize, addr,
                                 nToBeEvicted, ctxt->sBase.pCbkUserData);
        }
        // "Free" the least recently used page.
        UNSET_BIT(ctxt->pabitMappedPages, nOldPage);
        UNSET_BIT(ctxt->pabitRWMappedPages, nOldPage);
        // Free the old page.
        // Not sure how portable it is to do that that way.
        const void *const pRet =
            mmap(addr, ctxt->sBase.nPageSize, PROT_NONE,
                 MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        IGNORE_OR_ASSERT_IN_DEBUG(pRet == addr);
        // cppcheck-suppress memleak
    }
    ctxt->panLRUPageIndices[ctxt->iLRUStart] = iPage;
    ctxt->iLRUStart = (ctxt->iLRUStart + 1) % ctxt->nCacheMaxSizeInPages;
    if (ctxt->nLRUSize < ctxt->nCacheMaxSizeInPages)
    {
        ctxt->nLRUSize++;
    }
    SET_BIT(ctxt->pabitMappedPages, iPage);

    if (ctxt->sBase.bSingleThreadUsage)
    {
        if (opType == OP_STORE &&
            ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE)
        {
            // Let (and mark) the page writable since the instruction that
            // triggered the fault is a store.
            SET_BIT(ctxt->pabitRWMappedPages, iPage);
        }
        else if (ctxt->sBase.eAccessMode != VIRTUALMEM_READONLY)
        {
            const int nRet =
                mprotect(target_addr, ctxt->sBase.nPageSize, PROT_READ);
            IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
        }
    }
    else
    {
#ifdef HAVE_5ARGS_MREMAP
        (void)hRequesterThread;

        if (opType == OP_STORE &&
            ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE)
        {
            // Let (and mark) the page writable since the instruction that
            // triggered the fault is a store.
            SET_BIT(ctxt->pabitRWMappedPages, iPage);
        }
        else if (ctxt->sBase.eAccessMode != VIRTUALMEM_READONLY)
        {
            // Turn the temporary page read-only before remapping it.
            // Only turn it writtable when a new fault occurs (and the
            // mapping is writable).
            const int nRet =
                mprotect(pPageToFill, ctxt->sBase.nPageSize, PROT_READ);
            IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
        }
        /* Can now remap the pPageToFill onto the target page */
        const void *const pRet =
            mremap(pPageToFill, ctxt->sBase.nPageSize, ctxt->sBase.nPageSize,
                   MREMAP_MAYMOVE | MREMAP_FIXED, target_addr);
        IGNORE_OR_ASSERT_IN_DEBUG(pRet == target_addr);

#else
        if (ctxt->nThreads > 1)
        {
            /* Pause threads that share this mem view */
            CPLAtomicInc(&nWaitHelperThread);

            /* Install temporary SIGUSR1 signal handler */
            struct sigaction act, oldact;
            act.sa_sigaction = CPLVirtualMemSIGUSR1Handler;
            sigemptyset(&act.sa_mask);
            /* We don't want the sigsegv handler to be called when we are */
            /* running the sigusr1 handler */
            IGNORE_OR_ASSERT_IN_DEBUG(sigaddset(&act.sa_mask, SIGSEGV) == 0);
            act.sa_flags = 0;
            IGNORE_OR_ASSERT_IN_DEBUG(sigaction(SIGUSR1, &act, &oldact) == 0);

            for (int i = 0; i < ctxt->nThreads; i++)
            {
                if (ctxt->pahThreads[i] != hRequesterThread)
                {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
                    fprintfstderr("stopping thread %X\n", ctxt->pahThreads[i]);
#endif
                    IGNORE_OR_ASSERT_IN_DEBUG(
                        pthread_kill(ctxt->pahThreads[i], SIGUSR1) == 0);
                }
            }

            /* Wait that they are all paused */
            while (nCountThreadsInSigUSR1 != ctxt->nThreads - 1)
                usleep(1);

            /* Restore old SIGUSR1 signal handler */
            IGNORE_OR_ASSERT_IN_DEBUG(sigaction(SIGUSR1, &oldact, nullptr) ==
                                      0);

            int nRet = mprotect(target_addr, ctxt->sBase.nPageSize,
                                PROT_READ | PROT_WRITE);
            IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
            fprintfstderr("memcpying page %d\n", iPage);
#endif
            memcpy(target_addr, pPageToFill, ctxt->sBase.nPageSize);

            if (opType == OP_STORE &&
                ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE)
            {
                // Let (and mark) the page writable since the instruction that
                // triggered the fault is a store.
                SET_BIT(ctxt->pabitRWMappedPages, iPage);
            }
            else
            {
                nRet = mprotect(target_addr, ctxt->sBase.nPageSize, PROT_READ);
                IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
            }

            /* Wake up sleeping threads */
            CPLAtomicDec(&nWaitHelperThread);
            while (nCountThreadsInSigUSR1 != 0)
                usleep(1);

            IGNORE_OR_ASSERT_IN_DEBUG(
                munmap(pPageToFill, ctxt->sBase.nPageSize) == 0);
        }
        else
        {
            if (opType == OP_STORE &&
                ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE)
            {
                // Let (and mark) the page writable since the instruction that
                // triggered the fault is a store.
                SET_BIT(ctxt->pabitRWMappedPages, iPage);
            }
            else if (ctxt->sBase.eAccessMode != VIRTUALMEM_READONLY)
            {
                const int nRet2 =
                    mprotect(target_addr, ctxt->sBase.nPageSize, PROT_READ);
                IGNORE_OR_ASSERT_IN_DEBUG(nRet2 == 0);
            }
        }

        CPLReleaseMutex(ctxt->hMutexThreadArray);
#endif
    }
    // cppcheck-suppress memleak
}

/************************************************************************/
/*                    CPLVirtualMemGetOpTypeImm()                       */
/************************************************************************/

#if defined(__x86_64__) || defined(__i386__)
static OpType CPLVirtualMemGetOpTypeImm(GByte val_rip)
{
    OpType opType = OP_UNKNOWN;
    if ((/*val_rip >= 0x00 &&*/ val_rip <= 0x07) ||
        (val_rip >= 0x40 && val_rip <= 0x47))  // add $, (X)
        opType = OP_STORE;
    if ((val_rip >= 0x08 && val_rip <= 0x0f) ||
        (val_rip >= 0x48 && val_rip <= 0x4f))  // or $, (X)
        opType = OP_STORE;
    if ((val_rip >= 0x20 && val_rip <= 0x27) ||
        (val_rip >= 0x60 && val_rip <= 0x67))  // and $, (X)
        opType = OP_STORE;
    if ((val_rip >= 0x28 && val_rip <= 0x2f) ||
        (val_rip >= 0x68 && val_rip <= 0x6f))  // sub $, (X)
        opType = OP_STORE;
    if ((val_rip >= 0x30 && val_rip <= 0x37) ||
        (val_rip >= 0x70 && val_rip <= 0x77))  // xor $, (X)
        opType = OP_STORE;
    if ((val_rip >= 0x38 && val_rip <= 0x3f) ||
        (val_rip >= 0x78 && val_rip <= 0x7f))  // cmp $, (X)
        opType = OP_LOAD;
    return opType;
}
#endif

/************************************************************************/
/*                      CPLVirtualMemGetOpType()                        */
/************************************************************************/

// Don't need exhaustivity. It is just a hint for an optimization:
// If the fault occurs on a store operation, then we can directly put
// the page in writable mode if the mapping allows it.

#if defined(__x86_64__) || defined(__i386__)
static OpType CPLVirtualMemGetOpType(const GByte *rip)
{
    OpType opType = OP_UNKNOWN;

#if defined(__x86_64__) || defined(__i386__)
    switch (rip[0])
    {
        case 0x00: /* add %al,(%rax) */
        case 0x01: /* add %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x02: /* add (%rax),%al */
        case 0x03: /* add (%rax),%eax */
            opType = OP_LOAD;
            break;

        case 0x08: /* or %al,(%rax) */
        case 0x09: /* or %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x0a: /* or (%rax),%al */
        case 0x0b: /* or (%rax),%eax */
            opType = OP_LOAD;
            break;

        case 0x0f:
        {
            switch (rip[1])
            {
                case 0xb6: /* movzbl (%rax),%eax */
                case 0xb7: /* movzwl (%rax),%eax */
                case 0xbe: /* movsbl (%rax),%eax */
                case 0xbf: /* movswl (%rax),%eax */
                    opType = OP_LOAD;
                    break;
                default:
                    break;
            }
            break;
        }
        case 0xc6: /* movb $,(%rax) */
        case 0xc7: /* movl $,(%rax) */
            opType = OP_STORE;
            break;

        case 0x20: /* and %al,(%rax) */
        case 0x21: /* and %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x22: /* and (%rax),%al */
        case 0x23: /* and (%rax),%eax */
            opType = OP_LOAD;
            break;

        case 0x28: /* sub %al,(%rax) */
        case 0x29: /* sub %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x2a: /* sub (%rax),%al */
        case 0x2b: /* sub (%rax),%eax */
            opType = OP_LOAD;
            break;

        case 0x30: /* xor %al,(%rax) */
        case 0x31: /* xor %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x32: /* xor (%rax),%al */
        case 0x33: /* xor (%rax),%eax */
            opType = OP_LOAD;
            break;

        case 0x38: /* cmp %al,(%rax) */
        case 0x39: /* cmp %eax,(%rax) */
            opType = OP_LOAD;
            break;
        case 0x40:
        {
            switch (rip[1])
            {
                case 0x00: /* add %spl,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x02: /* add (%rax),%spl */
                    opType = OP_LOAD;
                    break;
                case 0x28: /* sub %spl,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x2a: /* sub (%rax),%spl */
                    opType = OP_LOAD;
                    break;
                case 0x3a: /* cmp (%rax),%spl */
                    opType = OP_LOAD;
                    break;
                case 0x8a: /* mov (%rax),%spl */
                    opType = OP_LOAD;
                    break;
                default:
                    break;
            }
            break;
        }
#if defined(__x86_64__)
        case 0x41: /* reg=%al/%eax, X=%r8 */
        case 0x42: /* reg=%al/%eax, X=%rax,%r8,1 */
        case 0x43: /* reg=%al/%eax, X=%r8,%r8,1 */
        case 0x44: /* reg=%r8b/%r8w, X = %rax */
        case 0x45: /* reg=%r8b/%r8w, X = %r8 */
        case 0x46: /* reg=%r8b/%r8w, X = %rax,%r8,1 */
        case 0x47: /* reg=%r8b/%r8w, X = %r8,%r8,1 */
        {
            switch (rip[1])
            {
                case 0x00: /* add regb,(X) */
                case 0x01: /* add regl,(X) */
                    opType = OP_STORE;
                    break;
                case 0x02: /* add (X),regb */
                case 0x03: /* add (X),regl */
                    opType = OP_LOAD;
                    break;
                case 0x0f:
                {
                    switch (rip[2])
                    {
                        case 0xb6: /* movzbl (X),regl */
                        case 0xb7: /* movzwl (X),regl */
                        case 0xbe: /* movsbl (X),regl */
                        case 0xbf: /* movswl (X),regl */
                            opType = OP_LOAD;
                            break;
                        default:
                            break;
                    }
                    break;
                }
                case 0x28: /* sub regb,(X) */
                case 0x29: /* sub regl,(X) */
                    opType = OP_STORE;
                    break;
                case 0x2a: /* sub (X),regb */
                case 0x2b: /* sub (X),regl */
                    opType = OP_LOAD;
                    break;
                case 0x38: /* cmp regb,(X) */
                case 0x39: /* cmp regl,(X) */
                    opType = OP_LOAD;
                    break;
                case 0x80: /* cmpb,... $,(X) */
                case 0x81: /* cmpl,... $,(X) */
                case 0x83: /* cmpl,... $,(X) */
                    opType = CPLVirtualMemGetOpTypeImm(rip[2]);
                    break;
                case 0x88: /* mov regb,(X) */
                case 0x89: /* mov regl,(X) */
                    opType = OP_STORE;
                    break;
                case 0x8a: /* mov (X),regb */
                case 0x8b: /* mov (X),regl */
                    opType = OP_LOAD;
                    break;
                case 0xc6: /* movb $,(X) */
                case 0xc7: /* movl $,(X) */
                    opType = OP_STORE;
                    break;
                case 0x84: /* test %al,(X) */
                    opType = OP_LOAD;
                    break;
                case 0xf6: /* testb $,(X) or notb (X) */
                case 0xf7: /* testl $,(X) or notl (X)*/
                {
                    if (rip[2] < 0x10) /* test (X) */
                        opType = OP_LOAD;
                    else /* not (X) */
                        opType = OP_STORE;
                    break;
                }
                default:
                    break;
            }
            break;
        }
        case 0x48: /* reg=%rax, X=%rax or %rax,%rax,1 */
        case 0x49: /* reg=%rax, X=%r8 or %r8,%rax,1 */
        case 0x4a: /* reg=%rax, X=%rax,%r8,1 */
        case 0x4b: /* reg=%rax, X=%r8,%r8,1 */
        case 0x4c: /* reg=%r8, X=%rax or %rax,%rax,1 */
        case 0x4d: /* reg=%r8, X=%r8 or %r8,%rax,1 */
        case 0x4e: /* reg=%r8, X=%rax,%r8,1 */
        case 0x4f: /* reg=%r8, X=%r8,%r8,1 */
        {
            switch (rip[1])
            {
                case 0x01: /* add reg,(X) */
                    opType = OP_STORE;
                    break;
                case 0x03: /* add (X),reg */
                    opType = OP_LOAD;
                    break;

                case 0x09: /* or reg,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x0b: /* or (%rax),reg */
                    opType = OP_LOAD;
                    break;
                case 0x0f:
                {
                    switch (rip[2])
                    {
                        case 0xc3: /* movnti reg,(X) */
                            opType = OP_STORE;
                            break;
                        default:
                            break;
                    }
                    break;
                }
                case 0x21: /* and reg,(X) */
                    opType = OP_STORE;
                    break;
                case 0x23: /* and (X),reg */
                    opType = OP_LOAD;
                    break;

                case 0x29: /* sub reg,(X) */
                    opType = OP_STORE;
                    break;
                case 0x2b: /* sub (X),reg */
                    opType = OP_LOAD;
                    break;

                case 0x31: /* xor reg,(X) */
                    opType = OP_STORE;
                    break;
                case 0x33: /* xor (X),reg */
                    opType = OP_LOAD;
                    break;

                case 0x39: /* cmp reg,(X) */
                    opType = OP_LOAD;
                    break;

                case 0x81:
                case 0x83:
                    opType = CPLVirtualMemGetOpTypeImm(rip[2]);
                    break;

                case 0x85: /* test reg,(X) */
                    opType = OP_LOAD;
                    break;

                case 0x89: /* mov reg,(X) */
                    opType = OP_STORE;
                    break;
                case 0x8b: /* mov (X),reg */
                    opType = OP_LOAD;
                    break;

                case 0xc7: /* movq $,(X) */
                    opType = OP_STORE;
                    break;

                case 0xf7:
                {
                    if (rip[2] < 0x10) /* testq $,(X) */
                        opType = OP_LOAD;
                    else /* notq (X) */
                        opType = OP_STORE;
                    break;
                }
                default:
                    break;
            }
            break;
        }
#endif
        case 0x66:
        {
            switch (rip[1])
            {
                case 0x01: /* add %ax,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x03: /* add (%rax),%ax */
                    opType = OP_LOAD;
                    break;
                case 0x0f:
                {
                    switch (rip[2])
                    {
                        case 0x2e: /* ucomisd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x6f: /* movdqa (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x7f: /* movdqa %xmm0,(%rax) */
                            opType = OP_STORE;
                            break;
                        case 0xb6: /* movzbw (%rax),%ax */
                            opType = OP_LOAD;
                            break;
                        case 0xe7: /* movntdq %xmm0,(%rax) */
                            opType = OP_STORE;
                            break;
                        default:
                            break;
                    }
                    break;
                }
                case 0x29: /* sub %ax,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x2b: /* sub (%rax),%ax */
                    opType = OP_LOAD;
                    break;
                case 0x39: /* cmp %ax,(%rax) */
                    opType = OP_LOAD;
                    break;
#if defined(__x86_64__)
                case 0x41: /* reg = %ax (or %xmm0), X = %r8 */
                case 0x42: /* reg = %ax (or %xmm0), X = %rax,%r8,1 */
                case 0x43: /* reg = %ax (or %xmm0), X = %r8,%r8,1 */
                case 0x44: /* reg = %r8w (or %xmm8), X = %rax */
                case 0x45: /* reg = %r8w (or %xmm8), X = %r8 */
                case 0x46: /* reg = %r8w (or %xmm8), X = %rax,%r8,1 */
                case 0x47: /* reg = %r8w (or %xmm8), X = %r8,%r8,1 */
                {
                    switch (rip[2])
                    {
                        case 0x01: /* add reg,(X) */
                            opType = OP_STORE;
                            break;
                        case 0x03: /* add (X),reg */
                            opType = OP_LOAD;
                            break;
                        case 0x0f:
                        {
                            switch (rip[3])
                            {
                                case 0x2e: /* ucomisd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x6f: /* movdqa (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x7f: /* movdqa reg,(X) */
                                    opType = OP_STORE;
                                    break;
                                case 0xb6: /* movzbw (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0xe7: /* movntdq reg,(X) */
                                    opType = OP_STORE;
                                    break;
                                default:
                                    break;
                            }
                            break;
                        }
                        case 0x29: /* sub reg,(X) */
                            opType = OP_STORE;
                            break;
                        case 0x2b: /* sub (X),reg */
                            opType = OP_LOAD;
                            break;
                        case 0x39: /* cmp reg,(X) */
                            opType = OP_LOAD;
                            break;
                        case 0x81: /* cmpw,... $,(X) */
                        case 0x83: /* cmpw,... $,(X) */
                            opType = CPLVirtualMemGetOpTypeImm(rip[3]);
                            break;
                        case 0x85: /* test reg,(X) */
                            opType = OP_LOAD;
                            break;
                        case 0x89: /* mov reg,(X) */
                            opType = OP_STORE;
                            break;
                        case 0x8b: /* mov (X),reg */
                            opType = OP_LOAD;
                            break;
                        case 0xc7: /* movw $,(X) */
                            opType = OP_STORE;
                            break;
                        case 0xf7:
                        {
                            if (rip[3] < 0x10) /* testw $,(X) */
                                opType = OP_LOAD;
                            else /* notw (X) */
                                opType = OP_STORE;
                            break;
                        }
                        default:
                            break;
                    }
                    break;
                }
#endif
                case 0x81: /* cmpw,... $,(%rax) */
                case 0x83: /* cmpw,... $,(%rax) */
                    opType = CPLVirtualMemGetOpTypeImm(rip[2]);
                    break;

                case 0x85: /* test %ax,(%rax) */
                    opType = OP_LOAD;
                    break;
                case 0x89: /* mov %ax,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0x8b: /* mov (%rax),%ax */
                    opType = OP_LOAD;
                    break;
                case 0xc7: /* movw $,(%rax) */
                    opType = OP_STORE;
                    break;
                case 0xf3:
                {
                    switch (rip[2])
                    {
                        case 0xa5: /* rep movsw %ds:(%rsi),%es:(%rdi) */
                            opType = OP_MOVS_RSI_RDI;
                            break;
                        default:
                            break;
                    }
                    break;
                }
                case 0xf7: /* testw $,(%rax) or notw (%rax) */
                {
                    if (rip[2] < 0x10) /* test */
                        opType = OP_LOAD;
                    else /* not */
                        opType = OP_STORE;
                    break;
                }
                default:
                    break;
            }
            break;
        }
        case 0x80: /* cmpb,... $,(%rax) */
        case 0x81: /* cmpl,... $,(%rax) */
        case 0x83: /* cmpl,... $,(%rax) */
            opType = CPLVirtualMemGetOpTypeImm(rip[1]);
            break;
        case 0x84: /* test %al,(%rax) */
        case 0x85: /* test %eax,(%rax) */
            opType = OP_LOAD;
            break;
        case 0x88: /* mov %al,(%rax) */
            opType = OP_STORE;
            break;
        case 0x89: /* mov %eax,(%rax) */
            opType = OP_STORE;
            break;
        case 0x8a: /* mov (%rax),%al */
            opType = OP_LOAD;
            break;
        case 0x8b: /* mov (%rax),%eax */
            opType = OP_LOAD;
            break;
        case 0xd9: /* 387 float */
        {
            if (rip[1] < 0x08) /* flds (%eax) */
                opType = OP_LOAD;
            else if (rip[1] >= 0x18 && rip[1] <= 0x20) /* fstps (%eax) */
                opType = OP_STORE;
            break;
        }
        case 0xf2: /* SSE 2 */
        {
            switch (rip[1])
            {
                case 0x0f:
                {
                    switch (rip[2])
                    {
                        case 0x10: /* movsd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x11: /* movsd %xmm0,(%rax) */
                            opType = OP_STORE;
                            break;
                        case 0x58: /* addsd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x59: /* mulsd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x5c: /* subsd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x5e: /* divsd (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        default:
                            break;
                    }
                    break;
                }
#if defined(__x86_64__)
                case 0x41: /* reg=%xmm0, X=%r8 or %r8,%rax,1 */
                case 0x42: /* reg=%xmm0, X=%rax,%r8,1 */
                case 0x43: /* reg=%xmm0, X=%r8,%r8,1 */
                case 0x44: /* reg=%xmm8, X=%rax or %rax,%rax,1*/
                case 0x45: /* reg=%xmm8, X=%r8 or %r8,%rax,1 */
                case 0x46: /* reg=%xmm8, X=%rax,%r8,1 */
                case 0x47: /* reg=%xmm8, X=%r8,%r8,1 */
                {
                    switch (rip[2])
                    {
                        case 0x0f:
                        {
                            switch (rip[3])
                            {
                                case 0x10: /* movsd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x11: /* movsd reg,(X) */
                                    opType = OP_STORE;
                                    break;
                                case 0x58: /* addsd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x59: /* mulsd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x5c: /* subsd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x5e: /* divsd (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                default:
                                    break;
                            }
                            break;
                        }
                        default:
                            break;
                    }
                    break;
                }
#endif
                default:
                    break;
            }
            break;
        }
        case 0xf3:
        {
            switch (rip[1])
            {
                case 0x0f: /* SSE 2 */
                {
                    switch (rip[2])
                    {
                        case 0x10: /* movss (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x11: /* movss %xmm0,(%rax) */
                            opType = OP_STORE;
                            break;
                        case 0x6f: /* movdqu (%rax),%xmm0 */
                            opType = OP_LOAD;
                            break;
                        case 0x7f: /* movdqu %xmm0,(%rax) */
                            opType = OP_STORE;
                            break;
                        default:
                            break;
                    }
                    break;
                }
#if defined(__x86_64__)
                case 0x41: /* reg=%xmm0, X=%r8 */
                case 0x42: /* reg=%xmm0, X=%rax,%r8,1 */
                case 0x43: /* reg=%xmm0, X=%r8,%r8,1 */
                case 0x44: /* reg=%xmm8, X = %rax */
                case 0x45: /* reg=%xmm8, X = %r8 */
                case 0x46: /* reg=%xmm8, X = %rax,%r8,1 */
                case 0x47: /* reg=%xmm8, X = %r8,%r8,1 */
                {
                    switch (rip[2])
                    {
                        case 0x0f: /* SSE 2 */
                        {
                            switch (rip[3])
                            {
                                case 0x10: /* movss (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x11: /* movss reg,(X) */
                                    opType = OP_STORE;
                                    break;
                                case 0x6f: /* movdqu (X),reg */
                                    opType = OP_LOAD;
                                    break;
                                case 0x7f: /* movdqu reg,(X) */
                                    opType = OP_STORE;
                                    break;
                                default:
                                    break;
                            }
                            break;
                        }
                        default:
                            break;
                    }
                    break;
                }
                case 0x48:
                {
                    switch (rip[2])
                    {
                        case 0xa5: /* rep movsq %ds:(%rsi),%es:(%rdi) */
                            opType = OP_MOVS_RSI_RDI;
                            break;
                        default:
                            break;
                    }
                    break;
                }
#endif
                case 0xa4: /* rep movsb %ds:(%rsi),%es:(%rdi) */
                case 0xa5: /* rep movsl %ds:(%rsi),%es:(%rdi) */
                    opType = OP_MOVS_RSI_RDI;
                    break;
                case 0xa6: /* repz cmpsb %es:(%rdi),%ds:(%rsi) */
                    opType = OP_LOAD;
                    break;
                default:
                    break;
            }
            break;
        }
        case 0xf6: /* testb $,(%rax) or notb (%rax) */
        case 0xf7: /* testl $,(%rax) or notl (%rax) */
        {
            if (rip[1] < 0x10) /* test */
                opType = OP_LOAD;
            else /* not */
                opType = OP_STORE;
            break;
        }
        default:
            break;
    }
#endif
    return opType;
}
#endif

/************************************************************************/
/*                    CPLVirtualMemManagerPinAddrInternal()             */
/************************************************************************/

static int
CPLVirtualMemManagerPinAddrInternal(CPLVirtualMemMsgToWorkerThread *msg)
{
    char wait_ready = '\0';
    char response_buf[4] = {};

    // Wait for the helper thread to be ready to process another request.
    while (true)
    {
        const int ret = static_cast<int>(
            read(pVirtualMemManager->pipefd_wait_thread[0], &wait_ready, 1));
        if (ret < 0 && errno == EINTR)
        {
            // NOP
        }
        else
        {
            IGNORE_OR_ASSERT_IN_DEBUG(ret == 1);
            break;
        }
    }

    // Pass the address that caused the fault to the helper thread.
    const ssize_t nRetWrite =
        write(pVirtualMemManager->pipefd_to_thread[1], msg, sizeof(*msg));
    IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == sizeof(*msg));

    // Wait that the helper thread has fixed the fault.
    while (true)
    {
        const int ret = static_cast<int>(
            read(pVirtualMemManager->pipefd_from_thread[0], response_buf, 4));
        if (ret < 0 && errno == EINTR)
        {
            // NOP
        }
        else
        {
            IGNORE_OR_ASSERT_IN_DEBUG(ret == 4);
            break;
        }
    }

    // In case the helper thread did not recognize the address as being
    // one that it should take care of, just rely on the previous SIGSEGV
    // handler (with might abort the process).
    return (memcmp(response_buf, MAPPING_FOUND, 4) == 0);
}

/************************************************************************/
/*                      CPLVirtualMemPin()                              */
/************************************************************************/

void CPLVirtualMemPin(CPLVirtualMem *ctxt, void *pAddr, size_t nSize,
                      int bWriteOp)
{
    if (ctxt->eType == VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED)
        return;

    CPLVirtualMemMsgToWorkerThread msg;

    memset(&msg, 0, sizeof(msg));
    msg.hRequesterThread = pthread_self();
    msg.opType = (bWriteOp) ? OP_STORE : OP_LOAD;

    char *pBase = reinterpret_cast<char *>(ALIGN_DOWN(pAddr, ctxt->nPageSize));
    const size_t n = (reinterpret_cast<char *>(pAddr) - pBase + nSize +
                      ctxt->nPageSize - 1) /
                     ctxt->nPageSize;
    for (size_t i = 0; i < n; i++)
    {
        msg.pFaultAddr = reinterpret_cast<char *>(pBase) + i * ctxt->nPageSize;
        CPLVirtualMemManagerPinAddrInternal(&msg);
    }
}

/************************************************************************/
/*                   CPLVirtualMemManagerSIGSEGVHandler()               */
/************************************************************************/

#if defined(__x86_64__)
#define REG_IP REG_RIP
#define REG_SI REG_RSI
#define REG_DI REG_RDI
#elif defined(__i386__)
#define REG_IP REG_EIP
#define REG_SI REG_ESI
#define REG_DI REG_EDI
#endif

// Must take care of only using "asynchronous-signal-safe" functions in a signal
// handler pthread_self(), read() and write() are such.  See:
// https://www.securecoding.cert.org/confluence/display/seccode/SIG30-C.+Call+only+asynchronous-safe+functions+within+signal+handlers
static void CPLVirtualMemManagerSIGSEGVHandler(int the_signal,
                                               siginfo_t *the_info,
                                               void *the_ctxt)
{
    CPLVirtualMemMsgToWorkerThread msg;

    memset(&msg, 0, sizeof(msg));
    msg.pFaultAddr = the_info->si_addr;
    msg.hRequesterThread = pthread_self();

#if defined(__x86_64__) || defined(__i386__)
    ucontext_t *the_ucontext = static_cast<ucontext_t *>(the_ctxt);
    const GByte *rip = reinterpret_cast<const GByte *>(
        the_ucontext->uc_mcontext.gregs[REG_IP]);
    msg.opType = CPLVirtualMemGetOpType(rip);
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
    fprintfstderr("at rip %p, bytes: %02x %02x %02x %02x\n", rip, rip[0],
                  rip[1], rip[2], rip[3]);
#endif
    if (msg.opType == OP_MOVS_RSI_RDI)
    {
        void *rsi =
            reinterpret_cast<void *>(the_ucontext->uc_mcontext.gregs[REG_SI]);
        void *rdi =
            reinterpret_cast<void *>(the_ucontext->uc_mcontext.gregs[REG_DI]);

#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
        fprintfstderr("fault=%p rsi=%p rsi=%p\n", msg.pFaultAddr, rsi, rdi);
#endif
        if (msg.pFaultAddr == rsi)
        {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
            fprintfstderr("load\n");
#endif
            msg.opType = OP_LOAD;
        }
        else if (msg.pFaultAddr == rdi)
        {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
            fprintfstderr("store\n");
#endif
            msg.opType = OP_STORE;
        }
    }
#ifdef DEBUG_VIRTUALMEM
    else if (msg.opType == OP_UNKNOWN)
    {
        static bool bHasWarned = false;
        if (!bHasWarned)
        {
            bHasWarned = true;
            fprintfstderr("at rip %p, unknown bytes: %02x %02x %02x %02x\n",
                          rip, rip[0], rip[1], rip[2], rip[3]);
        }
    }
#endif
#else
    msg.opType = OP_UNKNOWN;
#endif

#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
    fprintfstderr("entering handler for %X (addr=%p)\n", pthread_self(),
                  the_info->si_addr);
#endif

    if (the_info->si_code != SEGV_ACCERR)
    {
        pVirtualMemManager->oldact.sa_sigaction(the_signal, the_info, the_ctxt);
        return;
    }

    if (!CPLVirtualMemManagerPinAddrInternal(&msg))
    {
        // In case the helper thread did not recognize the address as being
        // one that it should take care of, just rely on the previous SIGSEGV
        // handler (with might abort the process).
        pVirtualMemManager->oldact.sa_sigaction(the_signal, the_info, the_ctxt);
    }

#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
    fprintfstderr("leaving handler for %X (addr=%p)\n", pthread_self(),
                  the_info->si_addr);
#endif
}

/************************************************************************/
/*                      CPLVirtualMemManagerThread()                    */
/************************************************************************/

static void CPLVirtualMemManagerThread(void * /* unused_param */)
{
    while (true)
    {
        char i_m_ready = 1;
        CPLVirtualMemVMA *ctxt = nullptr;
        bool bMappingFound = false;
        CPLVirtualMemMsgToWorkerThread msg;

        // Signal that we are ready to process a new request.
        ssize_t nRetWrite =
            write(pVirtualMemManager->pipefd_wait_thread[1], &i_m_ready, 1);
        IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == 1);

        // Fetch the address to process.
        const ssize_t nRetRead =
            read(pVirtualMemManager->pipefd_to_thread[0], &msg, sizeof(msg));
        IGNORE_OR_ASSERT_IN_DEBUG(nRetRead == sizeof(msg));

        // If CPLVirtualMemManagerTerminate() is called, it will use BYEBYE_ADDR
        // as a means to ask for our termination.
        if (msg.pFaultAddr == BYEBYE_ADDR)
            break;

        /* Lookup for a mapping that contains addr */
        CPLAcquireMutex(hVirtualMemManagerMutex, 1000.0);
        for (int i = 0; i < pVirtualMemManager->nVirtualMemCount; i++)
        {
            ctxt = pVirtualMemManager->pasVirtualMem[i];
            if (static_cast<char *>(msg.pFaultAddr) >=
                    static_cast<char *>(ctxt->sBase.pData) &&
                static_cast<char *>(msg.pFaultAddr) <
                    static_cast<char *>(ctxt->sBase.pData) + ctxt->sBase.nSize)
            {
                bMappingFound = true;
                break;
            }
        }
        CPLReleaseMutex(hVirtualMemManagerMutex);

        if (bMappingFound)
        {
            char *const start_page_addr = static_cast<char *>(
                ALIGN_DOWN(msg.pFaultAddr, ctxt->sBase.nPageSize));
            const int iPage =
                static_cast<int>((static_cast<char *>(start_page_addr) -
                                  static_cast<char *>(ctxt->sBase.pData)) /
                                 ctxt->sBase.nPageSize);

            if (iPage == ctxt->iLastPage)
            {
                // In case 2 threads try to access the same page concurrently it
                // is possible that we are asked to mapped the page again
                // whereas it is always mapped. However, if that number of
                // successive retries is too high, this is certainly a sign that
                // something else happen, like trying to write-access a
                // read-only page 100 is a bit of magic number. Rouault believes
                // it must be at least the number of concurrent threads. 100
                // seems to be really safe!
                ctxt->nRetry++;
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
                fprintfstderr("retry on page %d : %d\n", iPage, ctxt->nRetry);
#endif
                if (ctxt->nRetry >= 100)
                {
                    CPLError(CE_Failure, CPLE_AppDefined,
                             "CPLVirtualMemManagerThread: trying to "
                             "write into read-only mapping");
                    nRetWrite = write(pVirtualMemManager->pipefd_from_thread[1],
                                      MAPPING_NOT_FOUND, 4);
                    IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == 4);
                    break;
                }
                else if (msg.opType != OP_LOAD &&
                         ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE &&
                         !TEST_BIT(ctxt->pabitRWMappedPages, iPage))
                {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
                    fprintfstderr("switching page %d to write mode\n", iPage);
#endif
                    SET_BIT(ctxt->pabitRWMappedPages, iPage);
                    const int nRet =
                        mprotect(start_page_addr, ctxt->sBase.nPageSize,
                                 PROT_READ | PROT_WRITE);
                    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
                }
            }
            else
            {
                ctxt->iLastPage = iPage;
                ctxt->nRetry = 0;

                if (TEST_BIT(ctxt->pabitMappedPages, iPage))
                {
                    if (msg.opType != OP_LOAD &&
                        ctxt->sBase.eAccessMode == VIRTUALMEM_READWRITE &&
                        !TEST_BIT(ctxt->pabitRWMappedPages, iPage))
                    {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
                        fprintfstderr("switching page %d to write mode\n",
                                      iPage);
#endif
                        SET_BIT(ctxt->pabitRWMappedPages, iPage);
                        const int nRet =
                            mprotect(start_page_addr, ctxt->sBase.nPageSize,
                                     PROT_READ | PROT_WRITE);
                        IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
                    }
                    else
                    {
#if defined DEBUG_VIRTUALMEM && defined DEBUG_VERBOSE
                        fprintfstderr("unexpected case for page %d\n", iPage);
#endif
                    }
                }
                else
                {
                    void *const pPageToFill =
                        CPLVirtualMemGetPageToFill(ctxt, start_page_addr);

                    size_t nToFill = ctxt->sBase.nPageSize;
                    if (start_page_addr + nToFill >=
                        static_cast<char *>(ctxt->sBase.pData) +
                            ctxt->sBase.nSize)
                    {
                        nToFill = static_cast<char *>(ctxt->sBase.pData) +
                                  ctxt->sBase.nSize - start_page_addr;
                    }

                    ctxt->pfnCachePage(reinterpret_cast<CPLVirtualMem *>(ctxt),
                                       start_page_addr - static_cast<char *>(
                                                             ctxt->sBase.pData),
                                       pPageToFill, nToFill,
                                       ctxt->sBase.pCbkUserData);

                    // Now remap this page to its target address and
                    // register it in the LRU.
                    CPLVirtualMemAddPage(ctxt, start_page_addr, pPageToFill,
                                         msg.opType, msg.hRequesterThread);
                }
            }

            // Warn the segfault handler that we have finished our job.
            nRetWrite = write(pVirtualMemManager->pipefd_from_thread[1],
                              MAPPING_FOUND, 4);
            IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == 4);
        }
        else
        {
            // Warn the segfault handler that we have finished our job
            // but that the fault didn't occur in a memory range that
            // is under our responsibility.
            CPLError(CE_Failure, CPLE_AppDefined,
                     "CPLVirtualMemManagerThread: no mapping found");
            nRetWrite = write(pVirtualMemManager->pipefd_from_thread[1],
                              MAPPING_NOT_FOUND, 4);
            IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == 4);
        }
    }
}

/************************************************************************/
/*                       CPLVirtualMemManagerInit()                     */
/************************************************************************/

static bool CPLVirtualMemManagerInit()
{
    CPLMutexHolderD(&hVirtualMemManagerMutex);
    if (pVirtualMemManager != nullptr)
        return true;

    struct sigaction act;
    pVirtualMemManager = static_cast<CPLVirtualMemManager *>(
        VSI_MALLOC_VERBOSE(sizeof(CPLVirtualMemManager)));
    if (pVirtualMemManager == nullptr)
        return false;
    pVirtualMemManager->pasVirtualMem = nullptr;
    pVirtualMemManager->nVirtualMemCount = 0;
    int nRet = pipe(pVirtualMemManager->pipefd_to_thread);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
    nRet = pipe(pVirtualMemManager->pipefd_from_thread);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
    nRet = pipe(pVirtualMemManager->pipefd_wait_thread);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);

    // Install our custom SIGSEGV handler.
    act.sa_sigaction = CPLVirtualMemManagerSIGSEGVHandler;
    sigemptyset(&act.sa_mask);
    act.sa_flags = SA_SIGINFO;
    nRet = sigaction(SIGSEGV, &act, &pVirtualMemManager->oldact);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);

    // Starts the helper thread.
    pVirtualMemManager->hHelperThread =
        CPLCreateJoinableThread(CPLVirtualMemManagerThread, nullptr);
    if (pVirtualMemManager->hHelperThread == nullptr)
    {
        VSIFree(pVirtualMemManager);
        pVirtualMemManager = nullptr;
        return false;
    }
    return true;
}

/************************************************************************/
/*                      CPLVirtualMemManagerTerminate()                 */
/************************************************************************/

void CPLVirtualMemManagerTerminate(void)
{
    if (pVirtualMemManager == nullptr)
        return;

    CPLVirtualMemMsgToWorkerThread msg;
    msg.pFaultAddr = BYEBYE_ADDR;
    msg.opType = OP_UNKNOWN;
    memset(&msg.hRequesterThread, 0, sizeof(msg.hRequesterThread));

    // Wait for the helper thread to be ready.
    char wait_ready;
    const ssize_t nRetRead =
        read(pVirtualMemManager->pipefd_wait_thread[0], &wait_ready, 1);
    IGNORE_OR_ASSERT_IN_DEBUG(nRetRead == 1);

    // Ask it to terminate.
    const ssize_t nRetWrite =
        write(pVirtualMemManager->pipefd_to_thread[1], &msg, sizeof(msg));
    IGNORE_OR_ASSERT_IN_DEBUG(nRetWrite == sizeof(msg));

    // Wait for its termination.
    CPLJoinThread(pVirtualMemManager->hHelperThread);

    // Cleanup everything.
    while (pVirtualMemManager->nVirtualMemCount > 0)
        CPLVirtualMemFree(reinterpret_cast<CPLVirtualMem *>(
            pVirtualMemManager
                ->pasVirtualMem[pVirtualMemManager->nVirtualMemCount - 1]));
    CPLFree(pVirtualMemManager->pasVirtualMem);

    close(pVirtualMemManager->pipefd_to_thread[0]);
    close(pVirtualMemManager->pipefd_to_thread[1]);
    close(pVirtualMemManager->pipefd_from_thread[0]);
    close(pVirtualMemManager->pipefd_from_thread[1]);
    close(pVirtualMemManager->pipefd_wait_thread[0]);
    close(pVirtualMemManager->pipefd_wait_thread[1]);

    // Restore previous handler.
    sigaction(SIGSEGV, &pVirtualMemManager->oldact, nullptr);

    CPLFree(pVirtualMemManager);
    pVirtualMemManager = nullptr;

    CPLDestroyMutex(hVirtualMemManagerMutex);
    hVirtualMemManagerMutex = nullptr;
}

#else  // HAVE_VIRTUAL_MEM_VMA

CPLVirtualMem *CPLVirtualMemNew(
    size_t /* nSize */, size_t /* nCacheSize */, size_t /* nPageSizeHint */,
    int /* bSingleThreadUsage */, CPLVirtualMemAccessMode /* eAccessMode */,
    CPLVirtualMemCachePageCbk /* pfnCachePage */,
    CPLVirtualMemUnCachePageCbk /* pfnUnCachePage */,
    CPLVirtualMemFreeUserData /* pfnFreeUserData */, void * /* pCbkUserData */)
{
    CPLError(CE_Failure, CPLE_NotSupported,
             "CPLVirtualMemNew() unsupported on "
             "this operating system / configuration");
    return nullptr;
}

void CPLVirtualMemDeclareThread(CPLVirtualMem * /* ctxt */)
{
}

void CPLVirtualMemUnDeclareThread(CPLVirtualMem * /* ctxt */)
{
}

void CPLVirtualMemPin(CPLVirtualMem * /* ctxt */, void * /* pAddr */,
                      size_t /* nSize */, int /* bWriteOp */)
{
}

void CPLVirtualMemManagerTerminate(void)
{
}

#endif  // HAVE_VIRTUAL_MEM_VMA

#ifdef HAVE_MMAP

/************************************************************************/
/*                     CPLVirtualMemFreeFileMemoryMapped()              */
/************************************************************************/

static void CPLVirtualMemFreeFileMemoryMapped(CPLVirtualMem *ctxt)
{
    const size_t nMappingSize = ctxt->nSize +
                                static_cast<GByte *>(ctxt->pData) -
                                static_cast<GByte *>(ctxt->pDataToFree);
    const int nRet = munmap(ctxt->pDataToFree, nMappingSize);
    IGNORE_OR_ASSERT_IN_DEBUG(nRet == 0);
}

/************************************************************************/
/*                       CPLVirtualMemFileMapNew()                      */
/************************************************************************/

CPLVirtualMem *CPLVirtualMemFileMapNew(
    VSILFILE *fp, vsi_l_offset nOffset, vsi_l_offset nLength,
    CPLVirtualMemAccessMode eAccessMode,
    CPLVirtualMemFreeUserData pfnFreeUserData, void *pCbkUserData)
{
#if SIZEOF_VOIDP == 4
    if (nLength != static_cast<size_t>(nLength))
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "nLength = " CPL_FRMT_GUIB
                 " incompatible with 32 bit architecture",
                 nLength);
        return nullptr;
    }
    if (nOffset + CPLGetPageSize() !=
        static_cast<vsi_l_offset>(
            static_cast<off_t>(nOffset + CPLGetPageSize())))
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "nOffset = " CPL_FRMT_GUIB
                 " incompatible with 32 bit architecture",
                 nOffset);
        return nullptr;
    }
#endif

    int fd = static_cast<int>(
        reinterpret_cast<GUIntptr_t>(VSIFGetNativeFileDescriptorL(fp)));
    if (fd == 0)
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Cannot operate on a virtual file");
        return nullptr;
    }

    const off_t nAlignedOffset =
        static_cast<off_t>((nOffset / CPLGetPageSize()) * CPLGetPageSize());
    size_t nAlignment = static_cast<size_t>(nOffset - nAlignedOffset);
    size_t nMappingSize = static_cast<size_t>(nLength + nAlignment);

    // Need to ensure that the requested extent fits into the file size
    // otherwise SIGBUS errors will occur when using the mapping.
    vsi_l_offset nCurPos = VSIFTellL(fp);
    if (VSIFSeekL(fp, 0, SEEK_END) != 0)
        return nullptr;
    vsi_l_offset nFileSize = VSIFTellL(fp);
    if (nFileSize < nOffset + nLength)
    {
        if (eAccessMode != VIRTUALMEM_READWRITE)
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                     "Trying to map an extent outside of the file");
            CPL_IGNORE_RET_VAL(VSIFSeekL(fp, nCurPos, SEEK_SET));
            return nullptr;
        }
        else
        {
            char ch = 0;
            if (VSIFSeekL(fp, nOffset + nLength - 1, SEEK_SET) != 0 ||
                VSIFWriteL(&ch, 1, 1, fp) != 1)
            {
                CPLError(CE_Failure, CPLE_AppDefined,
                         "Cannot extend file to mapping size");
                CPL_IGNORE_RET_VAL(VSIFSeekL(fp, nCurPos, SEEK_SET));
                return nullptr;
            }
        }
    }
    if (VSIFSeekL(fp, nCurPos, SEEK_SET) != 0)
        return nullptr;

    CPLVirtualMem *ctxt = static_cast<CPLVirtualMem *>(
        VSI_CALLOC_VERBOSE(1, sizeof(CPLVirtualMem)));
    if (ctxt == nullptr)
        return nullptr;

    void *addr =
        mmap(nullptr, nMappingSize,
             eAccessMode == VIRTUALMEM_READWRITE ? PROT_READ | PROT_WRITE
                                                 : PROT_READ,
             MAP_SHARED, fd, nAlignedOffset);
    if (addr == MAP_FAILED)
    {
        int myerrno = errno;
        CPLError(CE_Failure, CPLE_AppDefined, "mmap() failed : %s",
                 strerror(myerrno));
        VSIFree(ctxt);
        // cppcheck thinks we are leaking addr.
        // cppcheck-suppress memleak
        return nullptr;
    }

    ctxt->eType = VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED;
    ctxt->nRefCount = 1;
    ctxt->eAccessMode = eAccessMode;
    ctxt->pData = static_cast<GByte *>(addr) + nAlignment;
    ctxt->pDataToFree = addr;
    ctxt->nSize = static_cast<size_t>(nLength);
    ctxt->nPageSize = CPLGetPageSize();
    ctxt->bSingleThreadUsage = false;
    ctxt->pfnFreeUserData = pfnFreeUserData;
    ctxt->pCbkUserData = pCbkUserData;

    return ctxt;
}

#else  // HAVE_MMAP

CPLVirtualMem *CPLVirtualMemFileMapNew(
    VSILFILE * /* fp */, vsi_l_offset /* nOffset */, vsi_l_offset /* nLength */,
    CPLVirtualMemAccessMode /* eAccessMode */,
    CPLVirtualMemFreeUserData /* pfnFreeUserData */, void * /* pCbkUserData */)
{
    CPLError(CE_Failure, CPLE_NotSupported,
             "CPLVirtualMemFileMapNew() unsupported on this "
             "operating system / configuration");
    return nullptr;
}

#endif  // HAVE_MMAP

/************************************************************************/
/*                         CPLGetPageSize()                             */
/************************************************************************/

size_t CPLGetPageSize(void)
{
#if defined(HAVE_MMAP) || defined(HAVE_VIRTUAL_MEM_VMA)
    return static_cast<size_t>(sysconf(_SC_PAGESIZE));
#else
    return 0;
#endif
}

/************************************************************************/
/*                   CPLIsVirtualMemFileMapAvailable()                  */
/************************************************************************/

int CPLIsVirtualMemFileMapAvailable(void)
{
#ifdef HAVE_MMAP
    return TRUE;
#else
    return FALSE;
#endif
}

/************************************************************************/
/*                        CPLVirtualMemFree()                           */
/************************************************************************/

void CPLVirtualMemFree(CPLVirtualMem *ctxt)
{
    if (ctxt == nullptr || --(ctxt->nRefCount) > 0)
        return;

    if (ctxt->pVMemBase != nullptr)
    {
        CPLVirtualMemFree(ctxt->pVMemBase);
        if (ctxt->pfnFreeUserData != nullptr)
            ctxt->pfnFreeUserData(ctxt->pCbkUserData);
        CPLFree(ctxt);
        return;
    }

#ifdef HAVE_MMAP
    if (ctxt->eType == VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED)
        CPLVirtualMemFreeFileMemoryMapped(ctxt);
#endif
#ifdef HAVE_VIRTUAL_MEM_VMA
    if (ctxt->eType == VIRTUAL_MEM_TYPE_VMA)
        CPLVirtualMemFreeFileMemoryMapped(
            reinterpret_cast<CPLVirtualMemVMA *>(ctxt));
#endif

    if (ctxt->pfnFreeUserData != nullptr)
        ctxt->pfnFreeUserData(ctxt->pCbkUserData);
    CPLFree(ctxt);
}

/************************************************************************/
/*                      CPLVirtualMemGetAddr()                          */
/************************************************************************/

void *CPLVirtualMemGetAddr(CPLVirtualMem *ctxt)
{
    return ctxt->pData;
}

/************************************************************************/
/*                     CPLVirtualMemIsFileMapping()                     */
/************************************************************************/

int CPLVirtualMemIsFileMapping(CPLVirtualMem *ctxt)
{
    return ctxt->eType == VIRTUAL_MEM_TYPE_FILE_MEMORY_MAPPED;
}

/************************************************************************/
/*                     CPLVirtualMemGetAccessMode()                     */
/************************************************************************/

CPLVirtualMemAccessMode CPLVirtualMemGetAccessMode(CPLVirtualMem *ctxt)
{
    return ctxt->eAccessMode;
}

/************************************************************************/
/*                      CPLVirtualMemGetPageSize()                      */
/************************************************************************/

size_t CPLVirtualMemGetPageSize(CPLVirtualMem *ctxt)
{
    return ctxt->nPageSize;
}

/************************************************************************/
/*                        CPLVirtualMemGetSize()                        */
/************************************************************************/

size_t CPLVirtualMemGetSize(CPLVirtualMem *ctxt)
{
    return ctxt->nSize;
}

/************************************************************************/
/*                   CPLVirtualMemIsAccessThreadSafe()                  */
/************************************************************************/

int CPLVirtualMemIsAccessThreadSafe(CPLVirtualMem *ctxt)
{
    return !ctxt->bSingleThreadUsage;
}

/************************************************************************/
/*                       CPLVirtualMemDerivedNew()                      */
/************************************************************************/

CPLVirtualMem *CPLVirtualMemDerivedNew(
    CPLVirtualMem *pVMemBase, vsi_l_offset nOffset, vsi_l_offset nSize,
    CPLVirtualMemFreeUserData pfnFreeUserData, void *pCbkUserData)
{
    if (nOffset + nSize > pVMemBase->nSize)
        return nullptr;

    CPLVirtualMem *ctxt = static_cast<CPLVirtualMem *>(
        VSI_CALLOC_VERBOSE(1, sizeof(CPLVirtualMem)));
    if (ctxt == nullptr)
        return nullptr;

    ctxt->eType = pVMemBase->eType;
    ctxt->nRefCount = 1;
    ctxt->pVMemBase = pVMemBase;
    pVMemBase->nRefCount++;
    ctxt->eAccessMode = pVMemBase->eAccessMode;
    ctxt->pData = static_cast<GByte *>(pVMemBase->pData) + nOffset;
    ctxt->pDataToFree = nullptr;
    ctxt->nSize = static_cast<size_t>(nSize);
    ctxt->nPageSize = pVMemBase->nPageSize;
    ctxt->bSingleThreadUsage = CPL_TO_BOOL(pVMemBase->bSingleThreadUsage);
    ctxt->pfnFreeUserData = pfnFreeUserData;
    ctxt->pCbkUserData = pCbkUserData;

    return ctxt;
}