[Bug libc/15661] posix_fallocate fallback code buggy and dangerous

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=15661

--- Comment #6 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Rich Felker from comment #1)
> If keeping the emulation is desirable, I do have one potential fix that
> would avoid the race condition. fork a child process (because this might
> SIGBUS if another thread or process truncates the file), possibly using
> clone to avoid the parent receiving signals from the child's termination. In
> the child, mmap the file and use atomic compare-and-swap on each page that
> should be allocated, with the comparison and new values being equal. This
> will touch each page with a write in a way that's non-destructive.

This assumes a unified page cache, which is probably not even true for all
Linux file systems.  I think it can still result in data loss.

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