[PATCH 5/6] ext4: write back partial-zeroed edges in WRITE_ZEROES

[yizhang089@gmail.com]

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

FALLOC_FL_WRITE_ZEROES requires that all blocks in the requested range
end up as written extents with zeroed content. For unaligned edges that
were partial-zeroed in dirty unwritten or delalloc state, the buffer
is left dirty while the underlying extent may not yet be converted to
written. As a result, a subsequent SYNC write to this range would still
trigger metadata changes, which violates the semantics of WRITE_ZEROES.

Fix this by calling filemap_write_and_wait_range() for partial-zeroed
edges to flush out the zeroed data and ensure the extent conversion
is complete.

Fixes: f4265b8d32c4 ("ext4: add FALLOC_FL_WRITE_ZEROES support")
Cc: stable@vger.kernel.org
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 fs/ext4/extents.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index c34505765521..c083703bf704 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4791,7 +4791,15 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 	if (IS_ALIGNED(offset | end, blocksize))
 		return ret;
 
-	if (((file->f_flags & O_SYNC) || IS_SYNC(inode)) && partial_zeroed) {
+	/*
+	 * In FALLOC_FL_WRITE_ZEROES mode, edges that have been partially
+	 * zeroed must be written back to ensure the entire zeroed range
+	 * is converted to the written state. In SYNC mode, writeback is
+	 * also required to persist the zeroed data to disk.
+	 */
+	if (partial_zeroed &&
+	    ((mode & FALLOC_FL_WRITE_ZEROES) ||
+	     (file->f_flags & O_SYNC) || IS_SYNC(inode))) {
 		ret = filemap_write_and_wait_range(inode->i_mapping, offset,
 						   end - 1);
 		if (ret)
-- 
2.53.0