On Mon, Apr 30, 2012 at 5:30 AM, Jan Kara <jack@xxxxxxx> wrote: > 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 Hello, Thank you revisit this. But as far as my remember is correct, this issue is NOT unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e. DIRECT_IO w/ multi thread process should not use fork(). -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
