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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

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>