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ext4: increase IO priority of fastcommit

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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@gmail.com>
Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20250827121812.1477634-1-sunjunchao@bytedance.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Julian Sun
2025-09-25 14:56:31 -04:00
committed by Theodore Ts'o
parent d6ace46c82
commit 46e75c56df
1 changed files with 1 additions and 1 deletions
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fs/ext4/fast_commit.c
+1 -1
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@@ -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 */