[PATCH v6 08/31] ext4: avoid unnecessary transaction in ext4_map_blocks() for unwritten extents

[Zhang Yi <yi.zhang@huaweicloud.com>]

From: Zhang Yi <yi.zhang@huawei.com>

When ext4_map_blocks() finds an unwritten extent in the extent cache and
the caller is willing to accept unwritten extents without conversion,
there is no need to start a journal transaction since no metadata update
is required. This avoids unnecessary transaction overhead in the
upcoming iomap writeback path when overwriting already-allocated
unwritten extents.

One thing to be careful about, as the comment in ext4_map_blocks()
states, if the flags contain EXT4_GET_BLOCKS_CREATE, the function will
mark @map as mapped.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 fs/ext4/inode.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c9dea4ca5caf..7a5c74af8ff3 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -809,14 +809,23 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 	 * Note that if blocks have been preallocated
 	 * ext4_ext_map_blocks() returns with buffer head unmapped
 	 */
-	if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED)
+	if (retval > 0) {
 		/*
-		 * If we need to convert extent to unwritten
-		 * we continue and do the actual work in
-		 * ext4_ext_map_blocks()
+		 * If we need to convert written extent to unwritten or
+		 * convert unwritten extent to written, continue and do
+		 * the actual work in ext4_ext_map_blocks().
 		 */
-		if (!(flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN))
+		if (map->m_flags & EXT4_MAP_MAPPED &&
+		    !(flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN))
 			goto out_handle;
+		if (map->m_flags & EXT4_MAP_UNWRITTEN &&
+		    (flags & EXT4_GET_BLOCKS_UNWRIT_EXT) &&
+		    !(flags & EXT4_GET_BLOCKS_CONVERT)) {
+			/* Contains EXT4_GET_BLOCKS_CREATE - mark mapped. */
+			map->m_flags |= EXT4_MAP_MAPPED;
+			goto out_handle;
+		}
+	}
 
 	if (!handle) {
 		handle = ext4_journal_start(inode, EXT4_HT_MAP_BLOCKS,
-- 
2.52.0