Re: [PATCH] Describe race of direct read and fork for unaligned buffers

[Jeff Moyer <jmoyer@redhat.com>]

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>