[PATCHv2 2/3] ext4: Extend ext4_overwrite_io() for dax path

[Ritesh Harjani <riteshh@xxxxxxxxxxxxx>]

DAX uses ->iomap_begin path which gets called to map/allocate
extents. In order to avoid starting journal txn where extent
allocation is not required, we need to check if ext4_map_blocks()
has returned any mapped extent entry.

Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
---
 fs/ext4/ext4.h |  2 ++
 fs/ext4/file.c | 14 +++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 42f5060f3cdf..8a1b468bfb49 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3232,6 +3232,8 @@ extern const struct dentry_operations ext4_dentry_ops;
 extern const struct inode_operations ext4_file_inode_operations;
 extern const struct file_operations ext4_file_operations;
 extern loff_t ext4_llseek(struct file *file, loff_t offset, int origin);
+extern bool ext4_overwrite_io(struct inode *inode, struct ext4_map_blocks *map,
+			      bool is_dax);
 
 /* inline.c */
 extern int ext4_get_max_inline_size(struct inode *inode);
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 84f73ed91af2..6c252498334b 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -188,7 +188,8 @@ ext4_extending_io(struct inode *inode, loff_t offset, size_t len)
 }
 
 /* Is IO overwriting allocated and initialized blocks? */
-static bool ext4_overwrite_io(struct inode *inode, struct ext4_map_blocks *map)
+bool ext4_overwrite_io(struct inode *inode, struct ext4_map_blocks *map,
+		       bool is_dax)
 {
 	unsigned int blkbits = inode->i_blkbits;
 	loff_t end = (map->m_lblk + map->m_len) << blkbits;
@@ -198,12 +199,19 @@ static bool ext4_overwrite_io(struct inode *inode, struct ext4_map_blocks *map)
 		return false;
 
 	err = ext4_map_blocks(NULL, inode, map, 0);
+
 	/*
+	 * In case of dax to avoid starting a transaction in ext4_iomap_begin()
+	 * we check if ext4_map_blocks() can return any mapped extent.
+	 *
 	 * 'err==len' means that all of the blocks have been preallocated,
 	 * regardless of whether they have been initialized or not. To exclude
 	 * unwritten extents, we need to check m_flags.
 	 */
-	return err == blklen && (map->m_flags & EXT4_MAP_MAPPED);
+	if (is_dax)
+		return err > 0 && (map->m_flags & EXT4_MAP_MAPPED);
+	else
+		return err == blklen && (map->m_flags & EXT4_MAP_MAPPED);
 }
 
 static ssize_t ext4_generic_write_checks(struct kiocb *iocb,
@@ -427,7 +435,7 @@ static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
 	 * in file_modified().
 	 */
 	if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
-	     !ext4_overwrite_io(inode, &map))) {
+	     !ext4_overwrite_io(inode, &map, false))) {
 		inode_unlock_shared(inode);
 		*ilock_shared = false;
 		inode_lock(inode);
-- 
2.25.4