Re: [PATCH v3] mm: mglru: fix stale batch updates after memcg reparenting

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

On 6/26/26 12:59 PM, Harry Yoo wrote:
> 
> 
> On 6/26/26 1:48 PM, Qi Zheng wrote:
>>
>>
>> On 6/26/26 12:43 PM, Harry Yoo wrote:
>>>
>>>
>>> On 6/26/26 11:27 AM, Qi Zheng wrote:
>>>> Hi Johannes,
>>>>
>>>> On 6/26/26 2:41 AM, Johannes Weiner wrote:
>>>>> On Thu, Jun 25, 2026 at 11:15:54PM +0800, Qi Zheng 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>
>>>>>> ---
>>>>>> 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 | 45 ++++++++++++++++++++++++++++++++++++++-------
>>>>>>     1 file changed, 38 insertions(+), 7 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>>> index 35c3bb15ae96..1ec8c23c72b9 100644
>>>>>> --- a/mm/vmscan.c
>>>>>> +++ b/mm/vmscan.c
>>>>>> @@ -3262,10 +3262,44 @@ 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();
>>>>>
>>>>> Where is this unlocked?
>>>>
>>>> The lruvec_unlock_irq() in reset_batch_size() will handle the unlocking.
>>>>
>>>>>
>>>>>> +    /*
>>>>>> +     * The memcg can be NULL when the memory controller is disabled.
>>>>>> +     * Otherwise, the caller keeps the memcg owning @lruvec alive.
>>>>>> +     */
>>>>>> +    if (!memcg || !css_is_dying(&memcg->css))
>>>>>> +        goto lock;
>>>>>> +
>>>>>> +    do {
>>>>>> +        memcg = parent_mem_cgroup(memcg);
>>>>>> +    } while (memcg && css_is_dying(&memcg->css));
>>>>>> +    lruvec = mem_cgroup_lruvec(memcg, pgdat);
>>>>>
>>>>>       while (unlikely(memcg && css_is_dying(&memcg->css))) {
>>>>>           memcg = parent_mem_cgroup(memcg);
>>>>>           lruvec = mem_cgroup_lruvec(memcg, pgdat);
>>>>
>>>> There is no need to acquire the lruvec before finding the first
>>>> non-dying memcg.
>>>
>>> struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>>> struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>>>
>>> rcu_read_lock()
>>>
>>> while (unlikely(memcg_is_dying(memcg)))
>>>           memcg = parent_mem_cgroup(memcg);
>>>
>>> lruvec = mem_cgroup_lruvec(memcg, pgdat);
>>
>> If the first memcg is already non-dying, there's no need to re-acquire
>> the lruvec. ;)
> 
> Oh, right :)
> 
> Hmm but I still think Johannes' suggestion makes the code cleaner.

I don't have a strong preference on which of the two coding styles is
more readable. BTW, is there any kernel documentation I could refer to
for this?

> Observing a dying cgroup should be rare anyway, it's worth focusing
> more on readability?

While it's rare to encounter consecutive dying memcgs, it can still
happen, right?

Thanks,
Qi

>