Re: [PATCH v3 24/30] mm: memcontrol: prepare for reparenting LRU pages for lruvec lock

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Jan 14, 2026 at 07:32:51PM +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>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/memcontrol.h | 45 +++++++++++++++++++----------
>  include/linux/swap.h       |  1 +
>  mm/compaction.c            | 29 +++++++++++++++----
>  mm/memcontrol.c            | 59 +++++++++++++++++++++-----------------
>  mm/swap.c                  |  4 +++
>  5 files changed, 91 insertions(+), 47 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 4b6f20dc694ba..26c3c0e375f58 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -742,7 +742,15 @@ 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.
> + * 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.
>   */
>  static inline struct lruvec *folio_lruvec(struct folio *folio)
>  {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 548e67dbf2386..a1573600d4188 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> diff --git a/mm/swap.c b/mm/swap.c
> index cb1148a92d8ec..7e53479ca1732 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -284,9 +286,11 @@ void lru_note_cost_unlock_irq(struct lruvec *lruvec, bool file,
>  		}
>  
>  		spin_unlock_irq(&lruvec->lru_lock);
> +		rcu_read_unlock();
>  		lruvec = parent_lruvec(lruvec);

It looks bit weird to call parent_lruvec(lruvec) outside RCU read lock
because the reason why it holds RCU read lock is to prevent release of
memory cgroup and its lruvec.

I guess this isn't broken (for now) because all callers of
lru_note_cost_unlock_irq() are holding a reference to the memcg?

>  		if (!lruvec)
>  			break;
> +		rcu_read_lock();
>  		spin_lock_irq(&lruvec->lru_lock);
>  	}
>  }

-- 
Cheers,
Harry / Hyeonggon