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

[Harry Yoo <harry@kernel.org>]

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On 6/26/26 3:24 PM, Qi Zheng wrote:
> 
> 
> 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?

I don't think there's a coding style guide that specifically
mentions this. Just thought it's cleaner because it merges if (...) goto
lock; and do-while into a single while loop.

>> 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?

But is worth saving a few instruction in a basic block that is
unlikely() to be executed?

I'm not a memcg maintainer myself, though. just my 2 cents.

-- 
Cheers,
Harry / Hyeonggon


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