Re: [PATCH v4 03/17] mm/memcg: add folio-based lruvec live helper

[Muchun Song <muchun.song@linux.dev>]

> On Oct 16, 2026, at 15:54, Kairui Song <ryncsn@gmail.com> wrote:
> 
> On Fri, Oct 16, 2026 at 3:39 PM Muchun Song <muchun.song@linux.dev> wrote:
>> 
>>> On Oct 16, 2026, at 02:15, Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> wrote:
>>> 
>>> From: Kairui Song <kasong@tencent.com>
>>> 
>>> Add a helper that resolves a stable lruvec for a folio under RCU
>>> without taking the lruvec lock.  It takes a folio directly so the
>>> lruvec lookup happens inside the RCU read-side critical section,
>>> which a lruvec-based interface cannot guarantee.
>>> 
>>> No functional change.
>>> 
>>> Tested-by: zhaozhengzhuo <zhaozhengzhuo@uniontech.com>
>>> Signed-off-by: Kairui Song <kasong@tencent.com>
>>> ---
>>> include/linux/memcontrol.h | 39 +++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 39 insertions(+)
>>> 
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index 74110a324f9e..829ab776ab44 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -1535,6 +1535,45 @@ static inline void lruvec_lock_irq(struct lruvec *lruvec)
>>> spin_lock_irq(&lruvec->lru_lock);
>>> }
>>> 
>>> +/**
>>> + * folio_lruvec_live_get - get a live lruvec for a folio under RCU
>>> + * @folio: the folio
>>> + *
>>> + * Computes @folio's lruvec and walks up to the nearest live ancestor
>>> + * if the folio's memcg is dying.  Paired with folio_lruvec_live_put().
>>> + * The result may be stale: RCU keeps it alive but does not pin @folio
>>> + * to it.  That is fine as the counters are fixed up on reparenting.
>>> + *
>>> + * Return: the live lruvec, with rcu_read_lock held.
>>> + */
>>> +static inline struct lruvec *folio_lruvec_live_get(struct folio *folio)
>>> +{
>>> +#ifdef CONFIG_MEMCG
>>> +     struct lruvec *lruvec;
>>> +     struct pglist_data *pgdat;
>>> +     struct mem_cgroup *memcg;
>>> +
>>> +     rcu_read_lock();
>>> +     lruvec = folio_lruvec(folio);
>>> +     pgdat = lruvec_pgdat(lruvec);
>>> +     memcg = lruvec_memcg(lruvec);
>>> +     while (unlikely(memcg && css_is_dying(&memcg->css))) {
>>> +             memcg = parent_mem_cgroup(memcg);
>>> +             lruvec = mem_cgroup_lruvec(memcg, pgdat);
>>> +     }
>>> +     return lruvec;
>> 
>> Hi Kairui,
>> 
> 
> Hi Muchun, thanks for the review.
> 
>> Actually, we don't have any means to guarantee that the memcg
>> to which the returned lruvec belongs will not become dying
>> after the return, right?
> 
> Yeah but Qi once mentioned that RCU prevents reparenting, right? Even
> if the memcg is dead after return, things happended to it
> (non-hieratical counter reparenting, ones in __lru_gen_reparent_memcg)
> will only occurs after RCU unlock so we can apply counter update to it
> without grabbing an ref, just hold the RCU.

Thanks for the info. I checked the code and confirmed that, aside from
keeping the memcg from being freed, RCU also ensures the memcg doesn't get
reparented. If your scenario requires mutual exclusion with the reparenting
operation, RCU can handle that.

Thanks,
Muchun

> 
>>> +#else
>>> +     return folio_lruvec(folio);
>> 
>> Then, I'm wondering why folio_lruvec() doesn't apply to the
>> memcg scenario.
>> 
>> Internally, folio_lruvec() obtains the memcg through
>> obj_cgroup_memcg(). So when the memcg becomes dying, it should
>> be reparented to the parent memcg within a short time.
> 
> I wasn't sure if grabbing RCU can ensure an already dead cgroup has
> not been reparented yet. If we apply the counter update to a already
> reparented lruvec, then the counter is leaked. This new helper is just
> a folio version of lruvec_live_lock_irq.
> 
> I got the idea from commit de4660898b7a, which says:
> 
> """
> .... check CSS_DYING under RCU before
> flushing the pending batch.  A non-dying memcg keeps the original lruvec
> stable against RCU-delayed offlining; a dying memcg redirects the deltas
> to the first non-dying ancestor.
> """
> 
> Applying counter update to a dead but not reparented lruvec is fine though.