linux-kernel - [PATCH v2 07/13] ext4: drop extent cache before splitting extent

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Message-ID: <20251121060811.1685783-8-yi.zhang@huaweicloud.com>
Date: Fri, 21 Nov 2025 14:08:05 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: linux-ext4@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	tytso@....edu,
	adilger.kernel@...ger.ca,
	jack@...e.cz,
	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
	yizhang089@...il.com,
	libaokun1@...wei.com,
	yangerkun@...wei.com
Subject: [PATCH v2 07/13] ext4: drop extent cache before splitting extent

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

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
       [UU|WWWWWWWWWW] on-disk extent     W: written extent
       [UU|UUUUUUUUUU] 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
       [UU|WWWWWWWWWW] on-disk extent     W: written extent
       [UU|WWWWWWWWUU] extent status tree
       [--|DDDDDDDDZZ]

Fix this issue by always remove cached extent status entry before
splitting extent.

Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 fs/ext4/extents.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 2b5aec3f8882..9bb80af4b5cf 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3367,6 +3367,12 @@ static struct ext4_ext_path *ext4_split_extent(handle_t *handle,
 	ee_len = ext4_ext_get_actual_len(ex);
 	unwritten = ext4_ext_is_unwritten(ex);

+	/*
+	 * Drop extent cache to prevent stale unwritten extents remaining
+	 * after zeroing out.
+	 */
+	ext4_es_remove_extent(inode, ee_block, ee_len);
+
 	/* Do not cache extents that are in the process of being modified. */
 	flags |= EXT4_EX_NOCACHE;

-- 
2.46.1