From: Muchun Song <songmuchun@bytedance.com>

In the near future, a folio will no longer pin its corresponding
memory cgroup. So an lruvec returned by folio_lruvec() could be
released without the rcu read lock or a reference to its memory
cgroup.

In the current patch, the rcu read lock is employed to safeguard
against the release of the lruvec in workingset_refault().

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/workingset.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 445fc634196d8..427ca1a5625e8 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -560,11 +560,12 @@ void workingset_refault(struct folio *folio, void *shadow)
 	 * locked to guarantee folio_memcg() stability throughout.
 	 */
 	nr = folio_nr_pages(folio);
+	rcu_read_lock();
 	lruvec = folio_lruvec(folio);
 	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
 
 	if (!workingset_test_recent(shadow, file, &workingset, true))
-		return;
+		goto out;
 
 	folio_set_active(folio);
 	workingset_age_nonresident(lruvec, nr);
@@ -580,6 +581,8 @@ void workingset_refault(struct folio *folio, void *shadow)
 		lru_note_cost_refault(folio);
 		mod_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + file, nr);
 	}
+out:
+	rcu_read_unlock();
 }
 
 /**
-- 
2.20.1

On Wed, Dec 17, 2025 at 03:27:43PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@bytedance.com>
> 
> In the near future, a folio will no longer pin its corresponding
> memory cgroup. So an lruvec returned by folio_lruvec() could be
> released without the rcu read lock or a reference to its memory
> cgroup.
> 
> In the current patch, the rcu read lock is employed to safeguard
> against the release of the lruvec in workingset_refault().
> 
> This serves as a preparatory measure for the reparenting of the
> LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  mm/workingset.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 445fc634196d8..427ca1a5625e8 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -560,11 +560,12 @@ void workingset_refault(struct folio *folio, void *shadow)
>  	 * locked to guarantee folio_memcg() stability throughout.
>  	 */
>  	nr = folio_nr_pages(folio);
> +	rcu_read_lock();
>  	lruvec = folio_lruvec(folio);
>  	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
>  
>  	if (!workingset_test_recent(shadow, file, &workingset, true))

This might sleep. You have to get a reference here and unlock RCU.

On 12/18/25 6:30 AM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:43PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@bytedance.com>
>>
>> In the near future, a folio will no longer pin its corresponding
>> memory cgroup. So an lruvec returned by folio_lruvec() could be
>> released without the rcu read lock or a reference to its memory
>> cgroup.
>>
>> In the current patch, the rcu read lock is employed to safeguard
>> against the release of the lruvec in workingset_refault().
>>
>> This serves as a preparatory measure for the reparenting of the
>> LRU pages.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>> ---
>>   mm/workingset.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/workingset.c b/mm/workingset.c
>> index 445fc634196d8..427ca1a5625e8 100644
>> --- a/mm/workingset.c
>> +++ b/mm/workingset.c
>> @@ -560,11 +560,12 @@ void workingset_refault(struct folio *folio, void *shadow)
>>   	 * locked to guarantee folio_memcg() stability throughout.
>>   	 */
>>   	nr = folio_nr_pages(folio);
>> +	rcu_read_lock();
>>   	lruvec = folio_lruvec(folio);
>>   	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
>>   
>>   	if (!workingset_test_recent(shadow, file, &workingset, true))
> 
> This might sleep. You have to get a reference here and unlock RCU.

Indeed, this may sleep in css_rstat_flush().

Will do the following:

get_mem_cgroup_from_folio(folio);
lruvec = mem_cgroup_lruvec(memcg, folio_pgdat(folio));

/* xxx */

mem_cgroup_put(memcg);