Patch "xfs: reject all unaligned direct writes to reflinked files" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

xfs: reject all unaligned direct writes to reflinked files

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xfs-reject-all-unaligned-direct-writes-to-reflinked-files.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

rom 54a4ef8af4e0dc5c983d17fcb9cf5fd25666d94e Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <[email protected]>
Date: Mon, 6 Feb 2017 13:00:54 -0800
Subject: xfs: reject all unaligned direct writes to reflinked files

From: Christoph Hellwig <[email protected]>

commit 54a4ef8af4e0dc5c983d17fcb9cf5fd25666d94e upstream.

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.

---
 fs/xfs/xfs_aops.c  |   45 ---------------------------------------------
 fs/xfs/xfs_file.c  |    9 +++++++++
 fs/xfs/xfs_trace.h |    2 +-
 3 files changed, 10 insertions(+), 46 deletions(-)

@@ -1263,44 +1263,6 @@ xfs_map_trim_size(
 	bh_result->b_size = mapping_size;
 }
 
-/* Bounce unaligned directio writes to the page cache. */
-static int
-xfs_bounce_unaligned_dio_write(
-	struct xfs_inode	*ip,
-	xfs_fileoff_t		offset_fsb,
-	struct xfs_bmbt_irec	*imap)
-{
-	struct xfs_bmbt_irec	irec;
-	xfs_fileoff_t		delta;
-	bool			shared;
-	bool			x;
-	int			error;
-
-	irec = *imap;
-	if (offset_fsb > irec.br_startoff) {
-		delta = offset_fsb - irec.br_startoff;
-		irec.br_blockcount -= delta;
-		irec.br_startblock += delta;
-		irec.br_startoff = offset_fsb;
-	}
-	error = xfs_reflink_trim_around_shared(ip, &irec, &shared, &x);
-	if (error)
-		return error;
-
-	/*
-	 * We're here because we're trying to do a directio write to a
-	 * region that isn't aligned to a filesystem block.  If any part
-	 * of the extent is shared, fall back to buffered mode to handle
-	 * the RMW.  This is done by returning -EREMCHG ("remote addr
-	 * changed"), which is caught further up the call stack.
-	 */
-	if (shared) {
-		trace_xfs_reflink_bounce_dio_write(ip, imap);
-		return -EREMCHG;
-	}
-	return 0;
-}
-
 STATIC int
 __xfs_get_blocks(
 	struct inode		*inode,

@@ -1438,13 +1400,6 @@ __xfs_get_blocks(
 	if (imap.br_startblock != HOLESTARTBLOCK &&
 	    imap.br_startblock != DELAYSTARTBLOCK &&
 	    (create || !ISUNWRITTEN(&imap))) {
-		if (create && direct && !is_cow) {
-			error = xfs_bounce_unaligned_dio_write(ip, offset_fsb,
-					&imap);
-			if (error)
-				return error;
-		}
-
 		xfs_map_buffer(inode, bh_result, &imap, offset);
 		if (ISUNWRITTEN(&imap))
 			set_buffer_unwritten(bh_result);

@@ -554,6 +554,15 @@ 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)) {
+			trace_xfs_reflink_bounce_dio_write(ip, iocb->ki_pos, count);
+			return -EREMCHG;
+		}
 		iolock = XFS_IOLOCK_EXCL;
 	} else {
 		iolock = XFS_IOLOCK_SHARED;

@@ -3353,7 +3353,7 @@ DEFINE_INODE_IREC_EVENT(xfs_reflink_conv
 DEFINE_RW_EVENT(xfs_reflink_reserve_cow);
 DEFINE_RW_EVENT(xfs_reflink_allocate_cow_range);
 
-DEFINE_INODE_IREC_EVENT(xfs_reflink_bounce_dio_write);
+DEFINE_SIMPLE_IO_EVENT(xfs_reflink_bounce_dio_write);
 DEFINE_IOMAP_EVENT(xfs_reflink_find_cow_mapping);
 DEFINE_INODE_IREC_EVENT(xfs_reflink_trim_irec);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/xfs-mark-speculative-prealloc-cow-fork-extents-unwritten.patch queue-4.9/xfs-fix-toctou-race-when-locking-an-inode-to-access-the-data-map.patch queue-4.9/xfs-use-iomap-new-flag-for-newly-allocated-delalloc-blocks.patch queue-4.9/xfs-reject-all-unaligned-direct-writes-to-reflinked-files.patch queue-4.9/xfs-allow-unwritten-extents-in-the-cow-fork.patch queue-4.9/xfs-tune-down-agno-asserts-in-the-bmap-code.patch queue-4.9/xfs-verify-free-block-header-fields.patch queue-4.9/xfs-check-for-obviously-bad-level-values-in-the-bmbt-root.patch queue-4.9/xfs-don-t-fail-xfs_extent_busy-allocation.patch queue-4.9/xfs-sync-eofblocks-scans-under-iolock-are-livelock-prone.patch queue-4.9/xfs-use-per-ag-reservations-for-the-finobt.patch queue-4.9/xfs-pull-up-iolock-from-xfs_free_eofblocks.patch queue-4.9/xfs-fail-_dir_open-when-readahead-fails.patch queue-4.9/xfs-update-ctime-and-mtime-on-clone-destinatation-inodes.patch queue-4.9/xfs-only-update-mount-resv-fields-on-success-in-__xfs_ag_resv_init.patch queue-4.9/xfs-use-xfs_icluster_size_fsb-to-calculate-inode-chunk-alignment.patch queue-4.9/xfs-only-reclaim-unwritten-cow-extents-periodically.patch queue-4.9/xfs-try-any-ag-when-allocating-the-first-btree-block-when-reflinking.patch queue-4.9/xfs-fix-and-streamline-error-handling-in-xfs_end_io.patch queue-4.9/xfs-fix-eofblocks-race-with-file-extending-async-dio-writes.patch