[Bug 230160] linuxulator doesn't implement madvise(MADV_DONTNEED) correctly

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230160

David Chisnall <theraven at FreeBSD.org> changed:

           What    |Removed                     |Added
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                 CC|                            |theraven at FreeBSD.org

--- Comment #1 from David Chisnall <theraven at FreeBSD.org> ---
It's actually worse than as described.  Linux's value for `MADV_DONTNEED` is 8,
which corresponds to FreeBSD's `MADV_NOCORE`, so we're not even getting the
FreeBSD `MADV_DONTNEED` behaviour.

This test program demonstrates the problem.  Compiled on Linux, it runs to
completion on a real Linux system and dies in the last assert on FreeBSD.

```
#include <sys/mman.h>
#include <assert.h>

int main(void)
{
        char *page = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_ANON |
MAP_PRIVATE, -1, 0);
        assert(page != MAP_FAILED);
        page[0] = 42;
        assert(page[0] == 42);
        madvise(page, 4096, MADV_DONTNEED);
        assert(page[0] == 0);
}
```

This `madvise` flag is commonly used by memory allocators to guarantee zeroed
memory for reuse.  It would be nice if we had a `MADV_ZERO` that did the same
thing as Linux's `MADV_DONTNEED` for shared memory as well as anonymous memory.

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