Good find, Eric.
Looks good to me. I guess, that can explain some problems I saw
on SLES10, and never had time to dig in it.
Seems to me, it's been that way for quite a while, though. This code was
introduced in 2.6.12. Prior to that, the code in finished_one_bio() did
following:
dio_complete(dio, dio->block_in_file <<
dio->blkbits,
dio->result);
where block_in_file is "current offset into the underlying file in
dio_block units", which is still the post-IO position, not the IO start
offset, isn't it?
- Felix
Eric Sandeen wrote:
We saw problems w/ xfs doing AIO+DIO into a sparse file.
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=217098
It seemed that xfs was doing "extent conversion" at the wrong offsets, so
written regions came up as unwritten (zeros) and stale data was exposed
in the region after the write. Thanks to Peter Backes for the very
nice testcase.
This also broke xen with blktap over xfs.
Here's what I found:
finished_one_bio calls dio_complete calls xfs_end_io_direct with an
offset, but:
offset = dio->iocb->ki_pos;
so this is the -current- post-IO position, not the IO start point that
dio_complete expects. So, xfs converts the the wrong region. Ouch!
XFS seems to be the only filesystem that uses the offset passed to the
end_io function, so only it is affected by this as near as I can tell.
However, the "short read" case is probably also wrong, as it is checking:
if ((dio->rw == READ) &&
((offset + transferred) > dio->i_size))
transferred = dio->i_size - offset;
but offset is the ending IO position in this case too.
This patch seems to fix it up.
Comments?
Thanks,
-Eric
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Index: linux-2.6.18/fs/direct-io.c
===================================================================
--- linux-2.6.18.orig/fs/direct-io.c
+++ linux-2.6.18/fs/direct-io.c
@@ -244,12 +244,12 @@ static void finished_one_bio(struct dio
*/
spin_unlock_irqrestore(&dio->bio_lock, flags);
- /* Check for short read case */
transferred = dio->result;
- offset = dio->iocb->ki_pos;
+ offset = dio->iocb->ki_pos - transferred;
+ /* Check for short read case */
if ((dio->rw == READ) &&
- ((offset + transferred) > dio->i_size))
+ (dio->iocb->ki_pos > dio->i_size))
transferred = dio->i_size - offset;
/* check for error in completion path */
-
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