[Bug 1926521] Re: QEMU-user ignores MADV_DONTNEED

[Laurent Vivier]

** Tags added: linux-user

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https://bugs.launchpad.net/bugs/1926521

Title:
  QEMU-user ignores MADV_DONTNEED

Status in QEMU:
  New

Bug description:
  There is comment int the code "This is a hint, so ignoring and returning 
success is ok"
  
https://github.com/qemu/qemu/blob/b1cffefa1b163bce9aebc3416f562c1d3886eeaa/linux-user/syscall.c#L11941

  But it seems incorrect with the current state of Linux

  "man madvise" or https://man7.org/linux/man-pages/man2/madvise.2.html
  says the following:
  >>  These advice values do not influence the semantics
  >>       of the application (except in the case of MADV_DONTNEED)

  >> After a successful MADV_DONTNEED operation, the semantics
  >> of memory access in the specified region are changed:
  >> subsequent accesses of pages in the range will succeed,
  >> but will result in either repopulating the memory contents
  >> from the up-to-date contents of the underlying mapped file
  >> (for shared file mappings, shared anonymous mappings, and
  >> shmem-based techniques such as System V shared memory
  >> segments) or zero-fill-on-demand pages for anonymous
  >> private mappings.

  Some applications use this behavior clear memory and it
  would be nice to be able to run them on QEMU without
  workarounds.

  Reproducer on "Debian 5.10.24 x86_64 GNU/Linux" as a host.

  
  ```
  #include "assert.h"
  #include "stdio.h"
  #include <sys/mman.h>
  #include <errno.h>

  int main() {
    char *P = (char *)mmap(0, 4096, PROT_READ | PROT_WRITE,
                           MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    assert(P);
    *P = 'A';
    while (madvise(P, 4096, MADV_DONTNEED) == -1 && errno == EAGAIN) {
    }
    assert(*P == 0);

    printf("OK\n");
  }

  /*
  gcc /tmp/madvice.c -o /tmp/madvice

  qemu-x86_64 /tmp/madvice
  madvice: /tmp/madvice.c:13: main: Assertion `*P == 0' failed.
  qemu: uncaught target signal 6 (Aborted) - core dumped
  Aborted

  /tmp/madvice
  OK

  
  */

  ```

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