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    SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
    Date
    Jan Kara <jack@suse.cz> writes:

    > This is a long standing problem (or a surprising feature) in our implementation
    > of get_user_pages() (used by direct IO). Since several attempts to fix it
    > failed (e.g.
    > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg06542.html, or
    > http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html refused in
    > http://comments.gmane.org/gmane.linux.kernel.mm/31569) and it's not completely
    > clear whether we really want to fix it given the costs, let's at least document
    > it.
    >
    > CC: mgorman@suse.de
    > CC: Jeff Moyer <jmoyer@redhat.com>
    > Signed-off-by: Jan Kara <jack@suse.cz>
    > ---
    >
    > --- a/man2/open.2 2012-04-27 00:07:51.736883092 +0200
    > +++ b/man2/open.2 2012-04-27 00:29:59.489892980 +0200
    > @@ -769,7 +769,12 @@
    > and the file offset must all be multiples of the logical block size
    > of the file system.
    > Under Linux 2.6, alignment to 512-byte boundaries
    > -suffices.
    > +suffices. However, if the user buffer is not page aligned and direct read
    > +runs in parallel with a
    > +.BR fork (2)
    > +of the reader process, it may happen that the read data is split between
    > +pages owned by the original process and its child. Thus effectively read
    > +data is corrupted.
    > .LP
    > The
    > .B O_DIRECT

    I think this sufficiently distills the problem. Thanks, Jan.

    Acked-by: Jeff Moyer <jmoyer@redhat.com>


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