test-MADV_DONTNEED.c

test-MADV_DONTNEED.c

// Gist to check possible issues with MADV_DONTNEED
// For example it does not supported by qemu user
// There is a patch for this [1], but it hasn't been applied.
//   [1]: https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg05422.html

#include <sys/mman.h>
#include <stdio.h>
#include <stddef.h>
#include <assert.h>
#include <string.h>

int main(int argc, char **argv)
{
    void *addr = mmap(NULL, 1<<16, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
    if (addr == MAP_FAILED) {
        perror("mmap");
        return 1;
    }
    memset(addr, 'A', 1<<16);

    if (!madvise(addr, 1<<16, MADV_DONTNEED)) {
        puts("MADV_DONTNEED does not return error. Check memory.");
        for (int i = 0; i < 1<<16; ++i) {
            assert(((unsigned char *)addr)[i] == 0);
        }
    } else {
        perror("madvise");
    }

    if (munmap(addr, 1<<16)) {
        perror("munmap");
        return 1;
    }

    return 0;
}

There is also https://patchwork.kernel.org/patch/10576637/ but it hasn't been applied