[PATCH] jbd2: Increase IO priority of checkpoint.

[PATCH] jbd2: Increase IO priority of checkpoint.

[Julian Sun]

In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
the priority of IOs initiated by jbd2 has been raised, exempting them
from WBT throttling.
Checkpoint is also a crucial operation of jbd2. While no serious issues
have been observed so far, it should still be reasonable to exempt
checkpoint from WBT throttling.

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
---
 fs/jbd2/checkpoint.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 38861ca04899..2d0719bf6d87 100644
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
 
 	blk_start_plug(&plug);
 	for (i = 0; i < *batch_count; i++)
-		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
+		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
 	blk_finish_plug(&plug);
 
 	for (i = 0; i < *batch_count; i++) {
-- 
2.20.1

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Jan Kara]

On Mon 25-08-25 20:53:39, Julian Sun wrote:
> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
> the priority of IOs initiated by jbd2 has been raised, exempting them
> from WBT throttling.
> Checkpoint is also a crucial operation of jbd2. While no serious issues
> have been observed so far, it should still be reasonable to exempt
> checkpoint from WBT throttling.
> 
> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>

Makes sense. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/jbd2/checkpoint.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 38861ca04899..2d0719bf6d87 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>  
>  	blk_start_plug(&plug);
>  	for (i = 0; i < *batch_count; i++)
> -		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
> +		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>  	blk_finish_plug(&plug);
>  
>  	for (i = 0; i < *batch_count; i++) {
> -- 
> 2.20.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Zhang Yi]

On 8/25/2025 8:53 PM, Julian Sun wrote:
> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
> the priority of IOs initiated by jbd2 has been raised, exempting them
> from WBT throttling.
> Checkpoint is also a crucial operation of jbd2. While no serious issues
> have been observed so far, it should still be reasonable to exempt
> checkpoint from WBT throttling.
> 
> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>

Looks reasonable to me. Thanks for the patch!

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
>  fs/jbd2/checkpoint.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 38861ca04899..2d0719bf6d87 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>  
>  	blk_start_plug(&plug);
>  	for (i = 0; i < *batch_count; i++)
> -		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
> +		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>  	blk_finish_plug(&plug);
>  
>  	for (i = 0; i < *batch_count; i++) {

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Ritesh Harjani]

Julian Sun <sunjunchao@bytedance.com> writes:

> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
> the priority of IOs initiated by jbd2 has been raised, exempting them
> from WBT throttling.
> Checkpoint is also a crucial operation of jbd2. While no serious issues
> have been observed so far, it should still be reasonable to exempt
> checkpoint from WBT throttling.
>

Interesting.. I was wondering whether we were able to observe any
throttling for jbd2 log writes or for jbd2 checkpoint?
Maybe It would have been nice, if we had some kind of data for this. 

BTW - does it make sense for fastcommit path too maybe for non-tail
fc write requests? I think it uses ext4_fc_submit_bh(). 

-ritesh


> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
> ---
>  fs/jbd2/checkpoint.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 38861ca04899..2d0719bf6d87 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>  
>  	blk_start_plug(&plug);
>  	for (i = 0; i < *batch_count; i++)
> -		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
> +		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>  	blk_finish_plug(&plug);
>  
>  	for (i = 0; i < *batch_count; i++) {
> -- 
> 2.20.1

Re: [External] Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Julian Sun]

在 2025/8/27 04:55, Ritesh Harjani (IBM) 写道:
> Julian Sun <sunjunchao@bytedance.com> writes:
> 
>> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
>> the priority of IOs initiated by jbd2 has been raised, exempting them
>> from WBT throttling.
>> Checkpoint is also a crucial operation of jbd2. While no serious issues
>> have been observed so far, it should still be reasonable to exempt
>> checkpoint from WBT throttling.
>>
> 
> Interesting.. I was wondering whether we were able to observe any
> throttling for jbd2 log writes or for jbd2 checkpoint?
> Maybe It would have been nice, if we had some kind of data for this.

Good idea. But AFAICS wbt lacks of such a obversation mechanism now..>
> BTW - does it make sense for fastcommit path too maybe for non-tail
> fc write requests? I think it uses ext4_fc_submit_bh().

Yeah, I think so.
After a rough check of the code, the following code paths may result in 
high latency or even task hangs:
   1. fastcommit io is throttled by wbt or other block layer qos policies.
   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.

Hi, Jan, please correct me if I'm missing anything.
> 
> -ritesh
> 
> 
>> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
>> ---
>>   fs/jbd2/checkpoint.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
>> index 38861ca04899..2d0719bf6d87 100644
>> --- a/fs/jbd2/checkpoint.c
>> +++ b/fs/jbd2/checkpoint.c
>> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>>   
>>   	blk_start_plug(&plug);
>>   	for (i = 0; i < *batch_count; i++)
>> -		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
>> +		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>>   	blk_finish_plug(&plug);
>>   
>>   	for (i = 0; i < *batch_count; i++) {
>> -- 
>> 2.20.1

Thanks,
-- 
Julian Sun <sunjunchao@bytedance.com>

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Julian Sun]

在 2025/8/27 04:55, Ritesh Harjani (IBM) 写道:
> Julian Sun <sunjunchao@bytedance.com> writes:
> 
>> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
>> the priority of IOs initiated by jbd2 has been raised, exempting them
>> from WBT throttling.
>> Checkpoint is also a crucial operation of jbd2. While no serious issues
>> have been observed so far, it should still be reasonable to exempt
>> checkpoint from WBT throttling.
>>
> 
> Interesting.. I was wondering whether we were able to observe any
> throttling for jbd2 log writes or for jbd2 checkpoint?
> Maybe It would have been nice, if we had some kind of data for this.

Good idea. But AFAICS wbt lacks of such a obversation mechanism now..>
> BTW - does it make sense for fastcommit path too maybe for non-tail
> fc write requests? I think it uses ext4_fc_submit_bh().

Yeah, I think so.
After a rough check of the code, the following code paths may result in 
high latency or even task hangs:
   1. fastcommit io is throttled by wbt or other block layer qos policies.
   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.

Hi, Jan, please correct me if I'm missing anything.
> 
> -ritesh
> 
> 
>> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
>> ---
>>   fs/jbd2/checkpoint.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
>> index 38861ca04899..2d0719bf6d87 100644
>> --- a/fs/jbd2/checkpoint.c
>> +++ b/fs/jbd2/checkpoint.c
>> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>>   
>>   	blk_start_plug(&plug);
>>   	for (i = 0; i < *batch_count; i++)
>> -		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
>> +		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>>   	blk_finish_plug(&plug);
>>   
>>   	for (i = 0; i < *batch_count; i++) {
>> -- 
>> 2.20.1

Thanks,
-- 
Julian Sun <sunjunchao@bytedance.com>

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Jan Kara]

On Wed 27-08-25 12:57:32, Julian Sun wrote:
> 在 2025/8/27 04:55, Ritesh Harjani (IBM) 写道:
> > Julian Sun <sunjunchao@bytedance.com> writes:
> > 
> > > In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
> > > the priority of IOs initiated by jbd2 has been raised, exempting them
> > > from WBT throttling.
> > > Checkpoint is also a crucial operation of jbd2. While no serious issues
> > > have been observed so far, it should still be reasonable to exempt
> > > checkpoint from WBT throttling.
> > > 
> > 
> > Interesting.. I was wondering whether we were able to observe any
> > throttling for jbd2 log writes or for jbd2 checkpoint?
> > Maybe It would have been nice, if we had some kind of data for this.
> 
> Good idea. But AFAICS wbt lacks of such a obversation mechanism now..>

Well, I guess Ritesh meant some test case which is reproducing the
situation where jbd2 log writes get stalled.

> > BTW - does it make sense for fastcommit path too maybe for non-tail
> > fc write requests? I think it uses ext4_fc_submit_bh().
> 
> Yeah, I think so.
> After a rough check of the code, the following code paths may result in high
> latency or even task hangs:
>   1. fastcommit io is throttled by wbt or other block layer qos policies.
>   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.
> 
> Hi, Jan, please correct me if I'm missing anything.

I think you're correct. In fact ext4_fc_commit() does modify current
process' io priority to match that of jbd2 thread so it would be logical
to match IO submission flags as well.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Julian Sun]

在 2025/8/27 19:15, Jan Kara 写道:
> On Wed 27-08-25 12:57:32, Julian Sun wrote:
>> 在 2025/8/27 04:55, Ritesh Harjani (IBM) 写道:
>>> Julian Sun <sunjunchao@bytedance.com> writes:
>>>
>>>> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
>>>> the priority of IOs initiated by jbd2 has been raised, exempting them
>>>> from WBT throttling.
>>>> Checkpoint is also a crucial operation of jbd2. While no serious issues
>>>> have been observed so far, it should still be reasonable to exempt
>>>> checkpoint from WBT throttling.
>>>>
>>>
>>> Interesting.. I was wondering whether we were able to observe any
>>> throttling for jbd2 log writes or for jbd2 checkpoint?
>>> Maybe It would have been nice, if we had some kind of data for this.
>>
>> Good idea. But AFAICS wbt lacks of such a obversation mechanism now..>
> 
> Well, I guess Ritesh meant some test case which is reproducing the
> situation where jbd2 log writes get stalled.

Ah, thanks for the clarification. It may be a little bit difficult to 
reproduce it manually, I’ll give it a shot but I’m not sure if I can do 
it..>
>>> BTW - does it make sense for fastcommit path too maybe for non-tail
>>> fc write requests? I think it uses ext4_fc_submit_bh().
>>
>> Yeah, I think so.
>> After a rough check of the code, the following code paths may result in high
>> latency or even task hangs:
>>    1. fastcommit io is throttled by wbt or other block layer qos policies.
>>    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.
>>
>> Hi, Jan, please correct me if I'm missing anything.
> 
> I think you're correct. In fact ext4_fc_commit() does modify current
> process' io priority to match that of jbd2 thread so it would be logical
> to match IO submission flags as well.

Thanks, I will send a patch to improve here.>
> 								Honza


-- 
Julian Sun <sunjunchao@bytedance.com>

Re: [PATCH] jbd2: Increase IO priority of checkpoint.

[Theodore Ts'o]

On Mon, 25 Aug 2025 20:53:39 +0800, Julian Sun wrote:
> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
> the priority of IOs initiated by jbd2 has been raised, exempting them
> from WBT throttling.
> Checkpoint is also a crucial operation of jbd2. While no serious issues
> have been observed so far, it should still be reasonable to exempt
> checkpoint from WBT throttling.
> 
> [...]

Applied, thanks!

[1/1] jbd2: Increase IO priority of checkpoint.
      commit: 0f3b05c12158ec7545fb336469ccce38c31c7f9f

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>