linux-ext4 - [PATCH] ext4: Increase IO priority of fastcommit.

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Message-Id: <20250827121812.1477634-1-sunjunchao@bytedance.com>
Date: Wed, 27 Aug 2025 20:18:12 +0800
From: Julian Sun <sunjunchao@...edance.com>
To: linux-ext4@...r.kernel.org
Cc: tytso@....edu,
	jack@...e.cz,
	harshadshirwadkar@...il.com,
	ritesh.list@...il.com
Subject: [PATCH] ext4: Increase IO priority of fastcommit.

The following code paths may result in high latency or even task hangs:
   1. fastcommit io is throttled by wbt.
   2. jbd2_fc_wait_bufs() might wait for a long time while
JBD2_FAST_COMMIT_ONGOING is set in journal->flags, and then
jbd2_journal_commit_transaction() waits for the
JBD2_FAST_COMMIT_ONGOING bit for a long time while holding the write
lock of j_state_lock.
   3. start_this_handle() waits for read lock of j_state_lock which
results in high latency or task hang.

Given the fact that ext4_fc_commit() already modifies the current
process' IO priority to match that of the jbd2 thread, it should be
reasonable to match jbd2's IO submission flags as well.

Suggested-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
Signed-off-by: Julian Sun <sunjunchao@...edance.com>
---
 fs/ext4/fast_commit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 42bee1d4f9f9..fa66b08de999 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -663,7 +663,7 @@ void ext4_fc_track_range(handle_t *handle, struct inode *inode, ext4_lblk_t star

 static void ext4_fc_submit_bh(struct super_block *sb, bool is_tail)
 {
-	blk_opf_t write_flags = REQ_SYNC;
+	blk_opf_t write_flags = JBD2_JOURNAL_REQ_FLAGS;
 	struct buffer_head *bh = EXT4_SB(sb)->s_fc_bh;

 	/* Add REQ_FUA | REQ_PREFLUSH only its tail */
-- 
2.20.1