linux-kernel - [PATCH v2 06/13] ext4: don't cache extent during splitting extent

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Message-ID: <20251121060811.1685783-7-yi.zhang@huaweicloud.com>
Date: Fri, 21 Nov 2025 14:08:04 +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 06/13] ext4: don't cache extent during splitting extent

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

Caching extents during the splitting process is risky, as it may result
in stale extents remaining in the status tree. Moreover, in most cases,
the corresponding extent block entries are likely already cached before
the split happens, making caching here not particularly useful.

Assume we have an unwritten extent, and then DIO writes the first half.

  [UUUUUUUUUUUUUUUU] on-disk extent        U: unwritten extent
  [UUUUUUUUUUUUUUUU] extent status tree
  |<-   ->| ----> dio write this range

First, when ext4_split_extent_at() splits this extent, it truncates the
existing extent and then inserts a new one. During this process, this
extent status entry may be shrunk, and calls to ext4_find_extent() and
ext4_cache_extents() may occur, which could potentially insert the
truncated range as a hole into the extent status tree. After the split
is completed, this hole is not replaced with the correct status.

  [UUUUUUU|UUUUUUUU] on-disk extent        U: unwritten extent
  [UUUUUUU|HHHHHHHH] extent status tree    H: hole

Then, the outer calling functions will not correct this remaining hole
extent either. Finally, if we perform a delayed buffer write on this
latter part, it will re-insert the delayed extent and cause an error in
space accounting.

In adition, if the unwritten extent cache is not shrunk during the
splitting, ext4_cache_extents() also conflicts with existing extents
when caching extents. In the future, we will add checks when caching
extents, which will trigger a warning. Therefore, Do not cache extents
that are being split.

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 19338f488550..2b5aec3f8882 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3199,6 +3199,9 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
 	BUG_ON((split_flag & EXT4_EXT_DATA_VALID1) &&
 	       (split_flag & EXT4_EXT_DATA_VALID2));

+	/* Do not cache extents that are in the process of being modified. */
+	flags |= EXT4_EX_NOCACHE;
+
 	ext_debug(inode, "logical block %llu\n", (unsigned long long)split);

 	ext4_ext_show_leaf(inode, path);
@@ -3364,6 +3367,9 @@ 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);

+	/* Do not cache extents that are in the process of being modified. */
+	flags |= EXT4_EX_NOCACHE;
+
 	if (map->m_lblk + map->m_len < ee_block + ee_len) {
 		split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT;
 		flags1 = flags | EXT4_GET_BLOCKS_PRE_IO;
-- 
2.46.1