[PATCH] ext4: don't order data when zeroing unwritten or delayed block

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

When zeroing out a written partial block, it is necessary to order the
data to prevent exposing stale data on disk. However, if the buffer is
unwritten or delayed, it is not allocated as written, so ordering the
data is not required. This can prevent strange and unnecessary ordered
writes when appending data across a region within a block.

Assume we have a 2K unwritten file on a filesystem with 4K blocksize,
and buffered write from 3K to 4K. Before this patch,
__ext4_block_zero_page_range() would add the range [2k,3k) to the
ordered range, and then the JBD2 commit process would write back this
block. However, it does nothing since the block is not mapped, this
folio will be redirtied and written back agian through the normal write
back process.

Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
---
 fs/ext4/inode.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index fa579e857baf..fc16a89903b9 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4104,9 +4104,13 @@ static int __ext4_block_zero_page_range(handle_t *handle,
 	if (ext4_should_journal_data(inode)) {
 		err = ext4_dirty_journalled_data(handle, bh);
 	} else {
-		err = 0;
 		mark_buffer_dirty(bh);
-		if (ext4_should_order_data(inode))
+		/*
+		 * Only the written block requires ordered data to prevent
+		 * exposing stale data.
+		 */
+		if (!buffer_unwritten(bh) && !buffer_delay(bh) &&
+		    ext4_should_order_data(inode))
 			err = ext4_jbd2_inode_add_write(handle, inode, from,
 					length);
 	}
-- 
2.52.0