linux-ext4 - [PATCH v2] ext4: don't order data when zeroing unwritten or delayed block

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Message-ID: <20251223011927.34042-1-yi.zhang@huaweicloud.com>
Date: Tue, 23 Dec 2025 09:19:27 +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,
	ojaswin@...ux.ibm.com,
	ritesh.list@...il.com,
	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
	yizhang089@...il.com,
	libaokun1@...wei.com,
	yangerkun@...wei.com,
	yukuai@...as.com
Subject: [PATCH v2] ext4: don't order data when zeroing unwritten or delayed block

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

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 as
written, this folio will be redirtied and written back agian through the
normal write back process.

Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
Reviewed-by: Jan Kara <jack@...e.cz>
---
 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 2e79b09fe2f0..f2d70c9af446 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4109,9 +4109,13 @@ 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