Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg offline

Zi Yan
2025-09-25T19:50:00Z

On 25 Sep 2025, at 15:35, David Hildenbrand wrote:

> On 25.09.25 08:11, Qi Zheng wrote:
>> Hi David,
>
> Hi :)
>
> [...]
>
>>>> +++ b/include/linux/mmzone.h
>>>> @@ -1346,6 +1346,7 @@ struct deferred_split {
>>>>       spinlock_t split_queue_lock;
>>>>       struct list_head split_queue;
>>>>       unsigned long split_queue_len;
>>>> +    bool is_dying;
>>>
>>> It's a bit weird to query whether the "struct deferred_split" is dying.
>>> Shouldn't this be a memcg property? (and in particular, not exist for
>>
>> There is indeed a CSS_DYING flag. But we must modify 'is_dying' under
>> the protection of the split_queue_lock, otherwise the folio may be added
>> back to the deferred_split of child memcg.
>
> Is there no way to reuse the existing mechanisms, and find a way to have the shrinker / queue locking sync against that?
>
> There is also the offline_css() function where we clear CSS_ONLINE. But it happens after calling ss->css_offline(css);

I see CSS_DYING will be set by kill_css() before offline_css() is called.
Probably the code can check CSS_DYING instead.

>
> Being able to query "is the memcg going offline" and having a way to sync against that would be probably cleanest.

So basically, something like:
1. at folio_split_queue_lock*() time, get folio’s memcg or
its parent memcg until there is no CSS_DYING set or CSS_ONLINE is set.
2. return the associated deferred_split_queue.

>
> I'll let all the memcg people comment on how that could be done best.
>
>>
>>> the pglist_data part where it might not make sense at all?).
>>
>> Maybe:
>>
>> #ifdef CONFIG_MEMCG
>> bool is_dying;
>> #endif
>>
>
> Still doesn't quite look like it would belong here :(
>
> Also, is "dying" really the right terminology? It's more like "going offline"?
>
> But then, the queue is not going offline, the memcg is ...
>
> --
> Cheers
>
> David / dhildenb

Best Regards,
Yan, Zi