FAILED: Patch "ext4: drop extent cache after doing PARTIAL_VALID1 zeroout" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 6d882ea3b0931b43530d44149b79fcd4ffc13030 Mon Sep 17 00:00:00 2001
From: Zhang Yi <[email protected]>
Date: Sat, 29 Nov 2025 18:32:38 +0800
Subject: [PATCH] ext4: drop extent cache after doing PARTIAL_VALID1 zeroout

When splitting an unwritten extent in the middle and converting it to initialized in ext4_split_extent() with the EXT4_EXT_MAY_ZEROOUT and EXT4_EXT_DATA_VALID2 flags set, it could leave a stale unwritten extent.

Assume we have an unwritten file and buffered write in the middle of it without dioread_nolock enabled, it will allocate blocks as written extent.

0 A B N
[UUUUUUUUUUUU] on-disk extent U: unwritten extent
[UUUUUUUUUUUU] extent status tree
[--DDDDDDDD--] D: valid data
|<- ->| ----> this range needs to be initialized

ext4_split_extent() first try to split this extent at B with EXT4_EXT_DATA_PARTIAL_VALID1 and EXT4_EXT_MAY_ZEROOUT flag set, but ext4_split_extent_at() failed to split this extent due to temporary lack of space. It zeroout B to N and leave the entire extent as unwritten.

0 A B N
[UUUUUUUUUUUU] on-disk extent
[UUUUUUUUUUUU] extent status tree
[--DDDDDDDDZZ] Z: zeroed data

ext4_split_extent() then try to split this extent at A with EXT4_EXT_DATA_VALID2 flag set. This time, it split successfully and leave an written extent from A to N.

0 A B N
[UUWWWWWWWWWW] on-disk extent W: written extent
[UUUUUUUUUUUU] extent status tree
[--DDDDDDDDZZ]

Finally ext4_map_create_blocks() only insert extent A to B to the extent status tree, and leave an stale unwritten extent in the status tree.

0 A B N
[UUWWWWWWWWWW] on-disk extent W: written extent
[UUWWWWWWWWUU] extent status tree
[--DDDDDDDDZZ]

Fix this issue by always cached extent status entry after zeroing out the second part.

Message-ID: <[email protected]>

---
 fs/ext4/extents.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

index be9fd2ab86679..1094e49234513 100644
@@ -3319,8 +3319,16 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
 			 * extent length and ext4_split_extent() split will the
 			 * first half again.
 			 */
-			if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1)
+			if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1) {
+				/*
+				 * Drop extent cache to prevent stale unwritten
+				 * extents remaining after zeroing out.
+				 */
+				ext4_es_remove_extent(inode,
+					le32_to_cpu(zero_ex.ee_block),
+					ext4_ext_get_actual_len(&zero_ex));
 				goto fix_extent_len;
+			}
 
 			/* update the extent length and mark as initialized */
 			ex->ee_len = cpu_to_le16(ee_len);

2.51.0