linux-ext4 - [PATCH] ext4: fast commit: avoid fs_reclaim inversion in perform

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Message-ID: <20251223131342.287864-1-me@linux.beauty>
Date: Tue, 23 Dec 2025 21:13:42 +0800
From: Li Chen <me@...ux.beauty>
To: "Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.cz>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>,
	linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Li Chen <me@...ux.beauty>
Subject: [PATCH] ext4: fast commit: avoid fs_reclaim inversion in perform_commit

lockdep reports a possible deadlock due to lock order inversion:

     CPU0                    CPU1
     ----                    ----
lock(fs_reclaim);
                             lock(&sbi->s_fc_lock);
                             lock(fs_reclaim);
lock(&sbi->s_fc_lock);

ext4_fc_perform_commit() holds s_fc_lock while writing the fast commit
log. Allocations here can enter reclaim and take fs_reclaim, inverting
with ext4_fc_del() which runs under fs_reclaim during inode eviction.
Wrap Step 6 in memalloc_nofs_save()/restore() so reclaim is skipped
while s_fc_lock is held.

Fixes: 6593714d67ba ("ext4: hold s_fc_lock while during fast commit")
Signed-off-by: Li Chen <me@...ux.beauty>
---
 fs/ext4/fast_commit.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 3bcdd4619de1..b0c458082997 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -1045,6 +1045,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
 	struct ext4_fc_head head;
 	struct inode *inode;
 	struct blk_plug plug;
+	unsigned int nofs;
 	int ret = 0;
 	u32 crc = 0;
 
@@ -1118,6 +1119,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
 		blkdev_issue_flush(journal->j_fs_dev);
 
 	blk_start_plug(&plug);
+	nofs = memalloc_nofs_save();
 	/* Step 6: Write fast commit blocks to disk. */
 	if (sbi->s_fc_bytes == 0) {
 		/*
@@ -1158,6 +1160,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
 
 out:
 	mutex_unlock(&sbi->s_fc_lock);
+	memalloc_nofs_restore(nofs);
 	blk_finish_plug(&plug);
 	return ret;
 }
-- 
2.52.0