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

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

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. ;)

Thanks,
Qi

> spin_lock_irq(&lruvec->lru_lock);
> 
> return lruvec;
> 
> should work?
> 
> if the memory controller is disabled, it's equivalent to:
> 
> rcu_read_lock();
> spin_lock_irq(&lruvec->lru_lock);
> return lruvec;
>