Re: [PATCH v2 23/28] mm: memcontrol: prepare for reparenting LRU pages for lruvec lock

[Qi Zheng <qi.zheng@linux.dev>]

On 12/18/25 9:00 PM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:47PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@bytedance.com>
>>
>> The following diagram illustrates how to ensure the safety of the folio
>> lruvec lock when LRU folios undergo reparenting.
>>
>> In the folio_lruvec_lock(folio) function:
>> ```
>>      rcu_read_lock();
>> retry:
>>      lruvec = folio_lruvec(folio);
>>      /* There is a possibility of folio reparenting at this point. */
>>      spin_lock(&lruvec->lru_lock);
>>      if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
>>          /*
>>           * The wrong lruvec lock was acquired, and a retry is required.
>>           * This is because the folio resides on the parent memcg lruvec
>>           * list.
>>           */
>>          spin_unlock(&lruvec->lru_lock);
>>          goto retry;
>>      }
>>
>>      /* Reaching here indicates that folio_memcg() is stable. */
>> ```
>>
>> In the memcg_reparent_objcgs(memcg) function:
>> ```
>>      spin_lock(&lruvec->lru_lock);
>>      spin_lock(&lruvec_parent->lru_lock);
>>      /* Transfer folios from the lruvec list to the parent's. */
>>      spin_unlock(&lruvec_parent->lru_lock);
>>      spin_unlock(&lruvec->lru_lock);
>> ```
>>
>> After acquiring the lruvec lock, it is necessary to verify whether
>> the folio has been reparented. If reparenting has occurred, the new
>> lruvec lock must be reacquired. During the LRU folio reparenting
>> process, the lruvec lock will also be acquired (this will be
>> implemented in a subsequent patch). Therefore, folio_memcg() remains
>> unchanged while the lruvec lock is held.
>>
>> Given that lruvec_memcg(lruvec) is always equal to folio_memcg(folio)
>> after the lruvec lock is acquired, the lruvec_memcg_debug() check is
>> redundant. Hence, it is removed.
>>
>> This patch serves as a preparation for the reparenting of LRU folios.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   include/linux/memcontrol.h | 26 ++++++++-----------
>>   mm/compaction.c            | 29 ++++++++++++++++-----
>>   mm/memcontrol.c            | 53 +++++++++++++++++++-------------------
>>   3 files changed, 61 insertions(+), 47 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 69c4bcfb3c3cd..85265b28c5d18 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -740,7 +740,11 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
>>    * folio_lruvec - return lruvec for isolating/putting an LRU folio
>>    * @folio: Pointer to the folio.
>>    *
>> - * This function relies on folio->mem_cgroup being stable.
>> + * The user should hold an rcu read lock to protect lruvec associated with
>> + * the folio from being released. But it does not prevent binding stability
>> + * between the folio and the returned lruvec from being changed to its parent
>> + * or ancestor (e.g. like folio_lruvec_lock() does that holds LRU lock to
>> + * prevent the change).
> 
> Can you please make this separate paragraphs to highlight the two
> distinct modes of access? Something like this:
> 
> Call with rcu_read_lock() held to ensure the lifetime of the returned
> lruvec. Note that this alone will NOT guarantee the stability of the
> folio->lruvec association; the folio can be reparented to an ancestor
> if this races with cgroup deletion.
> 
> Use folio_lruvec_lock() to ensure both lifetime and stability of the
> binding. Once a lruvec is locked, folio_lruvec() can be called on
> other folios, and their binding is stable if the returned lruvec
> matches the one the caller has locked. Useful for lock batching.

OK, will do in the next version.

> 
> Everything else looks good to me.
> 
> Thanks for putting so much effort into making these patches clean,
> well-documented, and the series so easy to review!
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks!