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;
}

unpatched qemu

$ qemu-x86_64-static /tmp/test-MADV_DONTNEED
MADV_DONTNEED does not return error. Check memory.
test-MADV_DONTNEED: /tmp/test-MADV_DONTNEED.c:19: main: Assertion `((unsigned char *)addr)[i] == 0' failed.
qemu: uncaught target signal 6 (Aborted) - core dumped
Aborted (core dumped)

patched qemu (by returning ENOSYS error)

qemu-x86_64 /tmp/test-MADV_DONTNEED
madvise: Success

patch for qemu to return ENOSYS

diff --git a/linux-user/syscall.c b/linux-user/syscall.c                                                                                 
index 897d20c076..5540792e0e 100644                                                                                                      
--- a/linux-user/syscall.c                                                                                                               
+++ b/linux-user/syscall.c                                                                                                               
@@ -11775,7 +11775,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,                                               
            turns private file-backed mappings into anonymous mappings.                                                                  
            This will break MADV_DONTNEED.                                                                                               
            This is a hint, so ignoring and returning success is ok.  */                                                                 
-        return 0;                                                                                                                       
+        return ENOSYS;                                                                                                                  
 #endif                                                                                                                                  
 #ifdef TARGET_NR_fcntl64                                                                                                                
     case TARGET_NR_fcntl64:                       

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