Re: [PATCH 2/2] jbd2: bound shrinker scans by examined checkpoint buffers

[Zhang Yi <yizhang089@gmail.com>]

On 7/13/2026 6:22 PM, Max Kellermann wrote:
> The jbd2 shrinker currently accounts only checkpoint buffers that it
> successfully releases against nr_to_scan.  Busy buffers therefore do not
> consume the scan budget.
> 
> If a checkpoint transaction contains mostly busy buffers, the shrinker
> can scan its entire checkpoint list while holding journal->j_list_lock.
> Large checkpoint lists can result in excessive lock hold times and leave
> other CPUs spinning on j_list_lock, causing soft lockups or RCU stalls.
> 
> Pass nr_to_scan into journal_shrink_one_cp_list() and decrement it for
> every buffer examined, including busy buffers.  Pass NULL from checkpoint
> cleanup paths so their existing full-list behavior is preserved.
> 
> This restores the scan-budget semantics that existed before
> journal_shrink_one_cp_list() was changed to always scan a complete
> checkpoint list.
> 
> Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>

This makes the semantic of nr_to_scan more clear.

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

> ---
>   fs/jbd2/checkpoint.c | 25 +++++++++++++------------
>   1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 5266017565ac..513273712010 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -358,15 +358,16 @@ int jbd2_cleanup_journal_tail(journal_t *journal)
>   /*
>    * journal_shrink_one_cp_list
>    *
> - * Find all the written-back checkpoint buffers in the given list
> - * and try to release them. If the whole transaction is released, set
> - * the 'released' parameter. Return the number of released checkpointed
> - * buffers.
> + * Find written-back checkpoint buffers in the given list and try to release
> + * them. If 'nr_to_scan' is set, scan at most that many buffers. If the whole
> + * transaction is released, set the 'released' parameter. Return the number of
> + * released checkpointed buffers.
>    *
>    * Called with j_list_lock held.
>    */
>   static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>   						enum jbd2_shrink_type type,
> +						unsigned long *nr_to_scan,
>   						bool *released)
>   {
>   	struct journal_head *last_jh;
> @@ -375,13 +376,15 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>   	int ret;
>   
>   	*released = false;
> -	if (!jh)
> +	if (!jh || (nr_to_scan && !*nr_to_scan))
>   		return 0;
>   
>   	last_jh = jh->b_cpprev;
>   	do {
>   		jh = next_jh;
>   		next_jh = jh->b_cpnext;
> +		if (nr_to_scan)
> +			(*nr_to_scan)--;
>   
>   		if (type == JBD2_SHRINK_DESTROY) {
>   			ret = __jbd2_journal_remove_checkpoint(jh);
> @@ -403,7 +406,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>   next:
>   		if (need_resched())
>   			break;
> -	} while (jh != last_jh);
> +	} while (jh != last_jh && (!nr_to_scan || *nr_to_scan));
>   
>   	return nr_freed;
>   }
> @@ -425,7 +428,6 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>   	tid_t first_tid = 0, last_tid = 0, next_tid = 0;
>   	tid_t tid = 0;
>   	unsigned long nr_freed = 0;
> -	unsigned long freed;
>   	bool first_set = false;
>   
>   again:
> @@ -458,10 +460,9 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>   		next_transaction = transaction->t_cpnext;
>   		tid = transaction->t_tid;
>   
> -		freed = journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> -						   JBD2_SHRINK_BUSY_SKIP, &released);
> -		nr_freed += freed;
> -		(*nr_to_scan) -= min(*nr_to_scan, freed);
> +		nr_freed += journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> +						       JBD2_SHRINK_BUSY_SKIP,
> +						       nr_to_scan, &released);
>   		if (*nr_to_scan == 0)
>   			break;
>   		if (need_resched() || spin_needbreak(&journal->j_list_lock))
> @@ -517,7 +518,7 @@ void __jbd2_journal_clean_checkpoint_list(journal_t *journal,
>   		transaction = next_transaction;
>   		next_transaction = transaction->t_cpnext;
>   		journal_shrink_one_cp_list(transaction->t_checkpoint_list,
> -					   type, &released);
> +					   type, NULL, &released);
>   		/*
>   		 * This function only frees up some memory if possible so we
>   		 * dont have an obligation to finish processing. Bail out if