Re: [PATCH v4] mm: mglru: fix stale batch updates after memcg reparenting

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jul 02, 2026 at 09:38:41AM +0800, Qi Zheng wrote:
> 
> 
> On 7/1/26 11:59 PM, Johannes Weiner wrote:
> > On Thu, Jul 02, 2026 at 12:36:42AM +0900, Harry Yoo wrote:
> >>
> >>
> >> On 7/1/26 11:57 PM, Usama Arif wrote:
> >>> On Wed,  1 Jul 2026 15:52:51 +0800 Qi Zheng <qi.zheng@linux.dev> wrote:
> >>>
> >>>> From: Qi Zheng <zhengqi.arch@bytedance.com>
> >>>>
> >>>> The mglru page table walker batches per-generation size deltas in
> >>>> walk->nr_pages while walking page tables without holding the lruvec lock.
> >>>> The reset_batch_size() later folds those deltas into walk->lruvec under
> >>>> the lruvec lock.
> >>>>
> >>>> The page table walker can run concurrently with the memcg reparenting path
> >>>> as follows:
> >>>>
> >>>> CPU0                           CPU1
> >>>> ====                           ====
> >>>>
> >>>> walk_mm
> >>>> --> walk_page_range
> >>>>      --> update_batch_size
> >>>>          --> walk->nr_pages += delta
> >>>>
> >>>>                                mem_cgroup_css_offline
> >>>>                                --> memcg_reparent_objcgs
> >>>>                                    --> lock lruvec
> >>>>                                        lru_gen_reparent_memcg
> >>>>                                        --> reparent child folios to parent
> >>>>                                        unlock lruvec
> >>>>
> >>>>      lock lruvec
> >>>>      reset_batch_size
> >>>>      --> child lrugen->nr_pages += delta
> >>>>
> >>>> This will trigger the following warning in lru_gen_exit_memcg():
> >>>>
> >>>> 	VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
> >>>> 				   sizeof(lruvec->lrugen.nr_pages)));
> >>>>
> >>>> And the user-visible impact of underestimated nr_pages in MGLRU was
> >>>> premature OOMs because MGLRU does not try to reclaim memory when nr_pages
> >>>> reaches zero, but there are still more pages.
> >>>>
> >>>> To fix it, make reset_batch_size() 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.
> >>>>
> >>>> Reported-by: Peiyang He <peiyang_he@smail.nju.edu.cn>
> >>>> Closes: https://lore.kernel.org/all/5A9E929D82717101+12fcf643-efb8-4b9a-a53a-1e28cc894f0b@smail.nju.edu.cn
> >>>> Fixes: f304652609ea ("mm: vmscan: prepare for reparenting MGLRU folios")
> >>>> Cc: <stable@vger.kernel.org>
> >>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> >>>> Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
> >>>> ---
> >>>> Changes in v4:
> >>>>   - re-implement lock_batch_lruvec() in a simpler way
> >>>>     (suggested by Johannes and Harry)
> >>>>   - collect Reviewed-by
> >>>>   - rebase onto the next-20260630
> >>>>
> >>>> Changes in v3:
> >>>>   - re-implement lock_batch_lruvec() by checking CSS_DYING under the RCU lock
> >>>>     (suggested by Harry)
> >>>>   - update the commit message (suggested by Harry)
> >>>>   - temporarily drop the previous Reviewed-by tags
> >>>>     (since the sync method has changed)
> >>>>   - rebase onto the next-20260624
> >>>>
> >>>> Changes in v2:
> >>>>   - update the commit message (pointed by Barry)
> >>>>   - collect Reviewed-by
> >>>>
> >>>>   mm/vmscan.c | 41 ++++++++++++++++++++++++++++++++++-------
> >>>>   1 file changed, 34 insertions(+), 7 deletions(-)
> >>>>
> >>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>>> index 35c3bb15ae96..ca1e2a870d51 100644
> >>>> --- a/mm/vmscan.c
> >>>> +++ b/mm/vmscan.c
> >>>> @@ -3262,10 +3262,40 @@ static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
> >>>>   	walk->nr_pages[new_gen][type][zone] += delta;
> >>>>   }
> >>>>   
> >>>> +#ifdef CONFIG_MEMCG
> >>>> +static struct lruvec *lock_batch_lruvec(struct lruvec *lruvec)
> >>>> +{
> >>>> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> >>>> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> >>>> +
> >>>> +	rcu_read_lock();
> >>>> +
> >>>> +	/*
> >>>> +	 * The memcg can be NULL when the memory controller is disabled.
> >>>> +	 * Otherwise, the caller keeps the memcg owning @lruvec alive.
> >>>> +	 */
> >>>> +	while (unlikely(memcg && css_is_dying(&memcg->css))) {
> >>>> +		memcg = parent_mem_cgroup(memcg);
> >>>> +		lruvec = mem_cgroup_lruvec(memcg, pgdat);
> >>>> +	}
> >>>> +
> >>>> +	spin_lock_irq(&lruvec->lru_lock);
> >>>
> >>> Do we need an rcu_read_unlock() here?
> >>
> >> lruvec_unlock_irq() does that.
> > 
> > Yeah, that tripped me up too. And it makes me think Shakeel was right
> > after all: this should live next to the other lruvec_lock() primitives.
> > 
> > Sure, MGLRU is the only user, but it's still much easier to understand
> > this if the code sits next to the rest of the API (and the unlock!).
> > 
> > lruvec_live_lock_irq()?
> 
> But lruvec_lock_irq() grabs the rcu lock too. :(

Yes, but it's self-explanatory if you put it with those definitions:

static inline void lruvec_lock_irq(struct lruvec *lruvec)
{
        rcu_read_lock();
        spin_lock_irq(&lruvec->lru_lock);
}

static struct lruvec *lruvec_live_lock_irq(struct lruvec *lruvec)
{
	struct mem_cgroup *memcg = lruvec_memcg(lruvec);

	rcu_read_lock();
	while (unlikely(memcg && css_is_dying(&memcg->css))) {
		memcg = parent_mem_cgroup(memcg);
		lruvec = mem_cgroup_lruvec(memcg, lruvec_pgdat(lruvec));
	}
	spin_lock_irq(&lruvec->lru_lock);
}

static inline void lruvec_unlock_irq(struct lruvec *lruvec)
{
        spin_unlock_irq(&lruvec->lru_lock);
        rcu_read_unlock();
}

etc.