Re: [PATCH v2 07/11] xfs: iomap CoW-based atomic write support

In cases of an atomic write occurs for misaligned or discontiguous disk
blocks, we will use a CoW-based method to issue the atomic write.

So, for that case, return -EAGAIN to request that the write be issued in
CoW atomic write mode. The dio write path should detect this, similar to
how misaligned regalar DIO writes are handled.

For normal HW-based mode, when the range which we are atomic writing to
covers a shared data extent, try to allocate a new CoW fork. However, if
we find that what we allocated does not meet atomic write requirements
in terms of length and alignment, then fallback on the CoW-based mode
for the atomic write.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/xfs_iomap.c | 72 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 69 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ab79f0080288..c5ecfafbba60 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c

@@ -795,6 +795,23 @@ imap_spans_range(
 	return true;
 }
 
+static bool
+imap_range_valid_for_atomic_write(

+	struct xfs_bmbt_irec	*imap,
+	xfs_fileoff_t		offset_fsb,
+	xfs_fileoff_t		end_fsb)
+{
+	/* Misaligned start block wrt size */
+	if (!IS_ALIGNED(imap->br_startblock, imap->br_blockcount))
+		return false;
+
+	/* Discontiguous or mixed extents */
+	if (!imap_spans_range(imap, offset_fsb, end_fsb))
+		return false;
+
+	return true;
+}
+
 static int
 xfs_direct_write_iomap_begin(
 	struct inode		*inode,

@@ -809,12 +826,20 @@ xfs_direct_write_iomap_begin(
 	struct xfs_bmbt_irec	imap, cmap;
 	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
 	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, length);
+	bool			atomic_cow = flags & IOMAP_ATOMIC_COW;
+	bool			atomic_hw = flags & IOMAP_ATOMIC_HW;
 	int			nimaps = 1, error = 0;
 	bool			shared = false;
 	u16			iomap_flags = 0;
+	xfs_fileoff_t		orig_offset_fsb;
+	xfs_fileoff_t		orig_end_fsb;
+	bool			needs_alloc;
 	unsigned int		lockmode;
 	u64			seq;
 
+	orig_offset_fsb = offset_fsb;

+
 	ASSERT(flags & (IOMAP_WRITE | IOMAP_ZERO));
 
 	if (xfs_is_shutdown(mp))

@@ -832,7 +857,7 @@ xfs_direct_write_iomap_begin(
 	 * COW writes may allocate delalloc space or convert unwritten COW
 	 * extents, so we need to make sure to take the lock exclusively here.
 	 */
-	if (xfs_is_cow_inode(ip))
+	if (xfs_is_cow_inode(ip) || atomic_cow)
 		lockmode = XFS_ILOCK_EXCL;
 	else
 		lockmode = XFS_ILOCK_SHARED;

@@ -857,6 +882,22 @@ xfs_direct_write_iomap_begin(
 	if (error)
 		goto out_unlock;
 
+	if (flags & IOMAP_ATOMIC_COW) {

+		error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
+				&lockmode,
+				(flags & IOMAP_DIRECT) || IS_DAX(inode), true);
+		/*
+		 * Don't check @shared. For atomic writes, we should error when
+		 * we don't get a CoW fork.

+		 */
+		if (error)
+			goto out_unlock;
+
+		end_fsb = imap.br_startoff + imap.br_blockcount;
+		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
+		goto out_found_cow;
+	}

+
 	if (imap_needs_cow(ip, flags, &imap, nimaps)) {
 		error = -EAGAIN;
 		if (flags & IOMAP_NOWAIT)

@@ -868,13 +909,38 @@ xfs_direct_write_iomap_begin(
 				(flags & IOMAP_DIRECT) || IS_DAX(inode), false);
 		if (error)
 			goto out_unlock;
-		if (shared)
+		if (shared) {
+			if (atomic_hw &&
+			    !imap_range_valid_for_atomic_write(&cmap,
+					orig_offset_fsb, orig_end_fsb)) {
+				error = -EAGAIN;
+				goto out_unlock;
+			}
 			goto out_found_cow;
+		}
 		end_fsb = imap.br_startoff + imap.br_blockcount;
 		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
 	}
 
-	if (imap_needs_alloc(inode, flags, &imap, nimaps))
+	needs_alloc = imap_needs_alloc(inode, flags, &imap, nimaps);
+
+	if (atomic_hw) {
+		error = -EAGAIN;
+		/*
+		 * Use CoW method for when we need to alloc > 1 block,
+		 * otherwise we might allocate less than what we need here and
+		 * have multiple mappings.
+		*/
+		if (needs_alloc && orig_end_fsb - orig_offset_fsb > 1)
+			goto out_unlock;
+
+		if (!imap_range_valid_for_atomic_write(&imap, orig_offset_fsb,
+						orig_end_fsb)) {