[PATCH 1/3] xfs: reject all unaligned direct writes to reflinked files

[Christoph Hellwig <hch@lst.de>]

We currently fall back from direct to buffered writes if we detect a
remaining shared extent in the iomap_begin callback.  But by the time
iomap_begin is called for the potentially unaligned end block we might
have already written most of the data to disk, which we'd now write
again using buffered I/O.  To avoid this reject all writes to reflinked
files before starting I/O so that we are guaranteed to only write the
data once.

The alternative would be to unshare the unaligned start and/or end block
before doing the I/O. I think that's doable, and will actually be
required to support reflinks on DAX file system.  But it will take a
little more time and I'd rather get rid of the double write ASAP.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_file.c  |  7 +++++++
 fs/xfs/xfs_iomap.c | 12 +-----------
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f5effa6..873cd42 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -532,6 +532,13 @@ xfs_file_dio_aio_write(
 	if ((iocb->ki_pos & mp->m_blockmask) ||
 	    ((iocb->ki_pos + count) & mp->m_blockmask)) {
 		unaligned_io = 1;
+
+		/*
+		 * We can't properly handle unaligned direct I/O to reflink
+		 * files yet, as we can't unshare a partial block.
+		 */
+		if (xfs_is_reflink_inode(ip))
+			return -EREMCHG;
 		iolock = XFS_IOLOCK_EXCL;
 	} else {
 		iolock = XFS_IOLOCK_SHARED;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 0d14742..78105db 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1026,17 +1026,7 @@ xfs_file_iomap_begin(
 		if (error)
 			goto out_unlock;
 
-		/*
-		 * We're here because we're trying to do a directio write to a
-		 * region that isn't aligned to a filesystem block.  If the
-		 * extent is shared, fall back to buffered mode to handle the
-		 * RMW.
-		 */
-		if (!(flags & IOMAP_REPORT) && shared) {
-			trace_xfs_reflink_bounce_dio_write(ip, &imap);
-			error = -EREMCHG;
-			goto out_unlock;
-		}
+		ASSERT((flags & IOMAP_REPORT) || !shared);
 	}
 
 	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
-- 
2.1.4