[PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Max Kellermann]

journal_shrink_one_cp_list() skips busy checkpoint buffers when called
with JBD2_SHRINK_BUSY_SKIP.  The continue statement on this path also
skips the need_resched() check at the end of the loop body.

Consequently, when a checkpoint list contains mostly busy buffers, the
shrinker can walk the entire list while holding journal->j_list_lock,
even when a reschedule has been requested.  Large checkpoint lists under
memory pressure can therefore cause long lock hold times and leave other
CPUs spinning on j_list_lock, resulting in soft lockups or RCU stalls.

Route the busy-buffer path through the need_resched() check so that the
shrinker can release j_list_lock and reschedule promptly, restoring
parity with the clean-buffer path, which already checks need_resched().
This does not change which checkpoint buffers are eligible for removal.

Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/jbd2/checkpoint.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 1508e2f54462..5266017565ac 100644
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -389,7 +389,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
 			ret = jbd2_journal_try_remove_checkpoint(jh);
 			if (ret < 0) {
 				if (type == JBD2_SHRINK_BUSY_SKIP)
-					continue;
+					goto next;
 				break;
 			}
 		}
@@ -400,6 +400,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
 			break;
 		}
 
+next:
 		if (need_resched())
 			break;
 	} while (jh != last_jh);
-- 
2.47.3

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

[Max Kellermann]

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>
---
 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
-- 
2.47.3

Re: [PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Jan Kara]

On Mon 13-07-26 12:22:28, Max Kellermann wrote:
> journal_shrink_one_cp_list() skips busy checkpoint buffers when called
> with JBD2_SHRINK_BUSY_SKIP.  The continue statement on this path also
> skips the need_resched() check at the end of the loop body.
> 
> Consequently, when a checkpoint list contains mostly busy buffers, the
> shrinker can walk the entire list while holding journal->j_list_lock,
> even when a reschedule has been requested.  Large checkpoint lists under
> memory pressure can therefore cause long lock hold times and leave other
> CPUs spinning on j_list_lock, resulting in soft lockups or RCU stalls.
> 
> Route the busy-buffer path through the need_resched() check so that the
> shrinker can release j_list_lock and reschedule promptly, restoring
> parity with the clean-buffer path, which already checks need_resched().
> This does not change which checkpoint buffers are eligible for removal.
> 
> Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>

Good catch! Thanks for fixing this. Feel free to add:

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

								Honza

> ---
>  fs/jbd2/checkpoint.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 1508e2f54462..5266017565ac 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -389,7 +389,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  			ret = jbd2_journal_try_remove_checkpoint(jh);
>  			if (ret < 0) {
>  				if (type == JBD2_SHRINK_BUSY_SKIP)
> -					continue;
> +					goto next;
>  				break;
>  			}
>  		}
> @@ -400,6 +400,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  			break;
>  		}
>  
> +next:
>  		if (need_resched())
>  			break;
>  	} while (jh != last_jh);
> -- 
> 2.47.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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

[Jan Kara]

On Mon 13-07-26 12:22:29, 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>

Looks good. Feel free to add:

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

								Honza

> ---
>  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
> -- 
> 2.47.3
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Zhang Yi]

On 7/13/2026 6:22 PM, Max Kellermann wrote:
> journal_shrink_one_cp_list() skips busy checkpoint buffers when called
> with JBD2_SHRINK_BUSY_SKIP.  The continue statement on this path also
> skips the need_resched() check at the end of the loop body.
> 
> Consequently, when a checkpoint list contains mostly busy buffers, the
> shrinker can walk the entire list while holding journal->j_list_lock,
> even when a reschedule has been requested.  Large checkpoint lists under
> memory pressure can therefore cause long lock hold times and leave other
> CPUs spinning on j_list_lock, resulting in soft lockups or RCU stalls.
> 
> Route the busy-buffer path through the need_resched() check so that the
> shrinker can release j_list_lock and reschedule promptly, restoring
> parity with the clean-buffer path, which already checks need_resched().
> This does not change which checkpoint buffers are eligible for removal.
> 
> Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>

Thanks for the fix! This looks good to me.

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

> ---
>   fs/jbd2/checkpoint.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 1508e2f54462..5266017565ac 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -389,7 +389,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>   			ret = jbd2_journal_try_remove_checkpoint(jh);
>   			if (ret < 0) {
>   				if (type == JBD2_SHRINK_BUSY_SKIP)
> -					continue;
> +					goto next;
>   				break;
>   			}
>   		}
> @@ -400,6 +400,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>   			break;
>   		}
>   
> +next:
>   		if (need_resched())
>   			break;
>   	} while (jh != last_jh);

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

[Zhang Yi]

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

Re: [PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Andreas Dilger]

On Jul 13, 2026, at 04:22, Max Kellermann <max.kellermann@ionos.com> wrote:
> 
> journal_shrink_one_cp_list() skips busy checkpoint buffers when called
> with JBD2_SHRINK_BUSY_SKIP.  The continue statement on this path also
> skips the need_resched() check at the end of the loop body.
> 
> Consequently, when a checkpoint list contains mostly busy buffers, the
> shrinker can walk the entire list while holding journal->j_list_lock,
> even when a reschedule has been requested.  Large checkpoint lists under
> memory pressure can therefore cause long lock hold times and leave other
> CPUs spinning on j_list_lock, resulting in soft lockups or RCU stalls.
> 
> Route the busy-buffer path through the need_resched() check so that the
> shrinker can release j_list_lock and reschedule promptly, restoring
> parity with the clean-buffer path, which already checks need_resched().
> This does not change which checkpoint buffers are eligible for removal.

One question I had about this patch is whether the list cleanup could get
into a livelock if it becomes too large?  If the iterator is processing
a large number of jh that cannot be freed, then `need_resched()` is hit,
if it restarts from the beginning of the list each time without processing
at least one jh it will not make forward progress.

Cheers, Andreas

> Fixes: b98dba273a0e ("jbd2: remove journal_clean_one_cp_list()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
> fs/jbd2/checkpoint.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 1508e2f54462..5266017565ac 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -389,7 +389,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  			ret = jbd2_journal_try_remove_checkpoint(jh);
>  			if (ret < 0) {
>  				if (type == JBD2_SHRINK_BUSY_SKIP)
> -					continue;
> +					goto next;
>  				break;
>  			}
>  		}
> @@ -400,6 +400,7 @@ static unsigned long journal_shrink_one_cp_list(struct journal_head *jh,
>  			break;
>  		}
>  
> +next:
>  		if (need_resched())
>  			break;
>  	} while (jh != last_jh);
> -- 
> 2.47.3
> 
> 


Cheers, Andreas

Re: [PATCH 1/2] jbd2: check need_resched() when skipping busy checkpoint buffers

[Max Kellermann]

On Thu, Jul 16, 2026 at 10:04 PM Andreas Dilger <adilger@dilger.ca> wrote:
> One question I had about this patch is whether the list cleanup could get
> into a livelock if it becomes too large?

It was a livelock *without* this patch, because all CPUs were busy
inside this piece of shrinker code; one of them holding j_list_lock,
all others spinning on that very spinlock (all CPUs busy), and actal
I/O was stalled seemingly forever.

> If the iterator is processing
> a large number of jh that cannot be freed, then `need_resched()` is hit,
> if it restarts from the beginning of the list each time without processing
> at least one jh it will not make forward progress.

If no progress is made, then this shrinker call just wasn't able to
free anything. That was the behavior before b98dba273a0e ("jbd2:
remove journal_clean_one_cp_list()") and my patch restores this
behavior. It's not a problem. It's faily normal for shrinkers to not
be able to free anything.

-- 
Max Kellermann
Principal Architect
Hosting Technology

cm4all | Im Mediapark 6a | 50670 Köln | Germany
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https://www.cm4all.com/impressum
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