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@xxxxxxx CC: Jeff Moyer <jmoyer@xxxxxxxxxx> Signed-off-by: Jan Kara <jack@xxxxxxx> --- --- 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>