Messages in this thread Patch in this message | | | From | Jan Kara <> | Subject | [PATCH] Describe race of direct read and fork for unaligned buffers | Date | Mon, 30 Apr 2012 11:30:07 +0200 |
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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
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