linux-ext4 - [PATCH] ext4: add missing down_write_data_sem in mext_move

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Message-Id: <20251208123713.1971068-1-sunjunchao@bytedance.com>
Date: Mon,  8 Dec 2025 20:37:13 +0800
From: Julian Sun <sunjunchao@...edance.com>
To: linux-ext4@...r.kernel.org
Cc: tytso@....edu,
	yi.zhang@...wei.com,
	jack@...e.cz
Subject: [PATCH] ext4: add missing down_write_data_sem in mext_move_extent().

Commit 962e8a01eab9 ("ext4: introduce mext_move_extent()") attempts to
call ext4_swap_extents() on the failure path to recover the swapped
extents, but fails to acquire locks for the two inode->i_data_sem,
triggering the BUG_ON statement in ext4_swap_extents().

This issue can be fixed by calling ext4_double_down_write_data_sem()
before ext4_swap_extents().

Signed-off-by: Julian Sun <sunjunchao@...edance.com>
Reported-by: syzbot+4ea6bd8737669b423aae@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/69368649.a70a0220.38f243.0093.GAE@google.com/
Fixes: 962e8a01eab9 ("ext4: introduce mext_move_extent()")
---
 fs/ext4/move_extent.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 0550fd30fd10..635fb8a52e0c 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -393,9 +393,11 @@ static int mext_move_extent(struct mext_data *mext, u64 *m_len)
 
 repair_branches:
 	ret2 = 0;
+	ext4_double_down_write_data_sem(orig_inode, donor_inode);
 	r_len = ext4_swap_extents(handle, donor_inode, orig_inode,
 				  mext->donor_lblk, orig_map->m_lblk,
 				  *m_len, 0, &ret2);
+	ext4_double_up_write_data_sem(orig_inode, donor_inode);
 	if (ret2 || r_len != *m_len) {
 		ext4_error_inode_block(orig_inode, (sector_t)(orig_map->m_lblk),
 				       EIO, "Unable to copy data block, data will be lost!");
-- 
2.39.5