Messages in this thread | | | Date | Mon, 18 Mar 2002 11:00:36 -0800 | From | Andrew Morton <> | Subject | Re: fadvise syscall? |
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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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