Linux's `mremap()`

main.c

#define _GNU_SOURCE

#include <unistd.h>
#include <sys/mman.h>

#include <assert.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#if !defined(MAP_ANONYMOUS)
    #if defined(MAP_ANON)
        #define MAP_ANONYMOUS MAP_ANON
    #else
        #error "anonymous pages not available"
    #endif
#endif

// Helpers
// -----------------------------------------------------------------------------

static size_t
get_page_size()
{
    return (size_t)sysconf(_SC_PAGESIZE);
}

static void
touch_pages(void *buf, size_t size, size_t page_size)
{
    size_t i;

    for (i = 0; i < size; i += page_size) {
        void *ptr;

        ptr = (void *)((uintptr_t)buf + i);
        *((size_t *)ptr) = 123;
    }
}

static void
check_pages(void *buf, size_t size, size_t page_size)
{
    size_t i;

    for (i = 0; i < size; i += page_size) {
        void *ptr;

        ptr = (void *)((uintptr_t)buf + i);
        if (*((size_t *)ptr) != 123) {
            printf("invalid data!\n");
            exit(0);
        }
    }

    printf("all good!\n");
}

static uint64_t
get_time()
{
    struct timespec time;

    if (clock_gettime(CLOCK_MONOTONIC, &time) != 0) {
        printf("failed to retrieve the current time\n");
        exit(0);
    }

    return (uint64_t)time.tv_sec * 1000000000ull + (uint64_t)time.tv_nsec;
}


// -----------------------------------------------------------------------------

int
main(void)
{
    size_t page_size;
    void *addr;
    size_t alloc_size;
    uint64_t start, stop;

    // -------------------------------------------------------------------------

    page_size = get_page_size();

    addr = (void *)0x10000000;

#if 0
    alloc_size = 0x40000000; // 1 GB
#else
    alloc_size = 0x00010000; // 64 KB
#endif

    printf("press any key to continue...\n");
    getchar();

    // -------------------------------------------------------------------------

    void *alloc_1 = mmap(
        (void *)0x10000000,
        alloc_size,
        PROT_READ | PROT_WRITE,
        MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
        -1,
        0
    );
    if (alloc_1 == NULL) {
        printf("alloc_1: mmap failed\n");
        exit(0);
    }

    touch_pages(alloc_1, alloc_size, page_size);

    printf("finished mapping alloc_1...\n");
    getchar();

    // -------------------------------------------------------------------------

    void *alloc_2 = mmap(
        (void *)((uintptr_t)alloc_1 + alloc_size),
        alloc_size,
        PROT_READ | PROT_WRITE,
        MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
        -1,
        0
    );
    if (alloc_2 == NULL) {
        printf("alloc_2: mmap failed\n");
        exit(0);
    }

    touch_pages(alloc_2, alloc_size, page_size);

    printf("finished mapping alloc_2...\n");
    getchar();

    // -------------------------------------------------------------------------

#if 1
    start = get_time();

    void *alloc_3 = mmap(
        (void *)((uintptr_t)alloc_2 + alloc_size),
        alloc_size * 2,
        PROT_READ | PROT_WRITE,
        MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
        -1,
        0
    );
    if (alloc_3 == NULL) {
        printf("alloc_3: mmap failed\n");
        exit(0);
    }

    memcpy(alloc_3, alloc_1, alloc_size);

    stop = get_time();

    printf(
        "finished mmapping + memcpying  (alloc_size = %zu KB)...\n",
        alloc_size / 1024);
    printf("elapsed: %f s\n", (double)(stop - start) * 1e-9);
#else
    start = get_time();

    void *alloc_3 = mremap(
        alloc_1,
        alloc_size,
        alloc_size * 2,
        MREMAP_MAYMOVE
    );
    if (alloc_3 == NULL) {
        printf("alloc_3: mremap failed\n");
        exit(0);
    }

    stop = get_time();

    printf(
        "finished remapping             (alloc_size = %zu KB)...\n",
        alloc_size / 1024);
    printf("elapsed: %f s\n", (double)(stop - start) * 1e-9);
#endif

    getchar();

    check_pages(alloc_3, alloc_size, page_size);

    // -------------------------------------------------------------------------

    printf("terminating...\n");
    getchar();

    return 0;
}