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QEMU-user ignores MADV_DONTNEED
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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