[PATCH] xfs: serialize unaligned dio writes against all other dio writes

[Brian Foster <bfoster@redhat.com>]

XFS applies more strict serialization constraints to unaligned
direct writes to accommodate things like direct I/O layer zeroing,
unwritten extent conversion, etc. Unaligned submissions acquire the
exclusive iolock and wait for in-flight dio to complete to ensure
multiple submissions do not race on the same block and cause data
corruption.

This generally works in the case of an aligned dio followed by an
unaligned dio, but the serialization is lost if I/Os occur in the
opposite order. If an unaligned write is submitted first and
immediately followed by an overlapping, aligned write, the latter
submits without the typical unaligned serialization barriers because
there is no indication of an unaligned dio still in-flight. This can
lead to unpredictable results.

To provide proper unaligned dio serialization, require that such
direct writes are always the only dio allowed in-flight at one time
for a particular inode. We already acquire the exclusive iolock and
drain pending dio before submitting the unaligned dio. Wait once
more after the dio submission to hold the iolock across the I/O and
prevent further submissions until the unaligned I/O completes. This
is heavy handed, but consistent with the current pre-submission
serialization for unaligned direct writes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---

I was originally going to deal with this problem by hacking in an inode
flag to track unaligned dio writes in-flight and use that to block any
follow on dio writes until cleared. Dave suggested we could use the
iolock to serialize by converting unaligned async dio writes to sync dio
writes and just letting the unaligned dio itself always block. That
seemed reasonable to me, but I morphed the approach slightly to just use
inode_dio_wait() because it seemed a bit cleaner. Thoughts?

Zorro,

You reproduced this problem originally. It addresses the problem in the
test case that reproduced for me. Care to confirm whether this patch
fixes the problem for you? Thanks.

Brian

 fs/xfs/xfs_file.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 770cc2edf777..8b2aaed82343 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -529,18 +529,19 @@ xfs_file_dio_aio_write(
 	count = iov_iter_count(from);
 
 	/*
-	 * If we are doing unaligned IO, wait for all other IO to drain,
-	 * otherwise demote the lock if we had to take the exclusive lock
-	 * for other reasons in xfs_file_aio_write_checks.
+	 * If we are doing unaligned IO, we can't allow any other IO in-flight
+	 * at the same time or we risk data corruption. Wait for all other IO to
+	 * drain, submit and wait for completion before we release the iolock.
+	 *
+	 * If the IO is aligned, demote the iolock if we had to take the
+	 * exclusive lock in xfs_file_aio_write_checks() for other reasons.
 	 */
 	if (unaligned_io) {
-		/* If we are going to wait for other DIO to finish, bail */
-		if (iocb->ki_flags & IOCB_NOWAIT) {
-			if (atomic_read(&inode->i_dio_count))
-				return -EAGAIN;
-		} else {
+		/* unaligned dio always waits, bail */
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return -EAGAIN;
+		else
 			inode_dio_wait(inode);
-		}
 	} else if (iolock == XFS_IOLOCK_EXCL) {
 		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
 		iolock = XFS_IOLOCK_SHARED;
@@ -548,6 +549,8 @@ xfs_file_dio_aio_write(
 
 	trace_xfs_file_direct_write(ip, count, iocb->ki_pos);
 	ret = iomap_dio_rw(iocb, from, &xfs_iomap_ops, xfs_dio_write_end_io);
+	if (unaligned_io && !is_sync_kiocb(iocb))
+		inode_dio_wait(inode);
 out:
 	xfs_iunlock(ip, iolock);
 
-- 
2.17.2