Re: fadvise syscall?

[Andrew Morton <akpm@zip.com.au>]

Richard Gooch wrote:
> 
> Andrew Morton writes:
> > Note that it applies to a file descriptor.  If
> > posix_fadvise(FADV_DONTNEED) is called against a file descriptor,
> > and someone else has an fd open against the same file, that other
> > user gets their foot shot off.  That's OK.
> 
> Let me verify that I understand what you're saying. Process A and B
> independently open the file. The file is already in the cache (because
> other processes regularly read this file). Process A is slowly reading
> stuff. Process B does FADV_DONTNEED on the whole file. The pages are
> dropped.
> 
> You're saying this is OK? How about this DoS attack:
>         int fd = open ("/lib/libc.so", O_RDONLY, 0);
>         while (1) {
>                 posix_fadvise (fd, 0, 0, FADVISE_DONTNEED);
>                 sleep (1);
>         }
> 
> Let me see that disc head move! Wheeee!
> 

POSIX_FADV_DONTNEED could only unmap pages from the caller's
VMA's, so the problem would only affect other processes which
share the same mm - CLONE_MM threads.

If some other process has a reference on the pages then they
wouldn't get unmapped as a result of this.  It's the same
as madvise(MADV_DONTNEED).

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