Re: [PATCH] mm: vmscan: protect shrinker idr replace with CONFIG_MEMCG

[Kirill Tkhai]

On 05.12.2019 13:00, Kirill Tkhai wrote:
> On 05.12.2019 12:43, Michal Hocko wrote:
>> On Thu 05-12-19 11:23:28, Kirill Tkhai wrote:
>>> On 04.12.2019 22:16, Yang Shi wrote:
>>>> Since commit 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 ("mm: shrinker:
>>>> make shrinker not depend on memcg kmem"), shrinkers' idr is protected by
>>>> CONFIG_MEMCG instead of CONFIG_MEMCG_KMEM, so it makes no sense to
>>>> protect shrinker idr replace with CONFIG_MEMCG_KMEM.
>>>>
>>>> Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>>>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>>> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
>>>> Cc: Roman Gushchin <guro@xxxxxx>
>>>> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>>>
>>> It looks like that in CONFIG_SLOB case we do not even call some shrinkers
>>> for subordinate mem cgroups (i.e., we don't call deferred_split_shrinker),
>>> since they never become completely registered.
>>>
>>> Fixes: 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 ("mm: shrinker: make shrinker not depend on memcg kmem")
>>
>> I am confused. Why the Fixes tag? Nothing should be really broken with
>> KMEM config guard right?
> 
> idr_replace() is disabled in CONFIG_MEMCG && CONFIG_SLOB case, and this is
> wrong.
> 
> 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 goes in the series, which enables
> shrinker_idr infrastructure for huge_memory.c's deferred_split_shrinker
> in CONFIG_MEMCG case. Previously, all SHRINKER_MEMCG_AWARE shrinkers were
> based on LRUs, and they remain to base of CONFIG_MEMCG_KMEM.
> But deferred_split_shrinker is an exception.
> 
> In CONFIG_MEMCG && CONFIG_SLOB case, shrinker_idr contains only shrinker,
> and it is deferred_split_shrinker. But it is never actually called, since
> idr_replace() is never compiled. deferred_split_shrinker all the time is
> staying in half-registered state, and it's never called for subordinate
> mem cgroups.
> 
> So, this is a BUG, and this should go to stable.

The visible effect is that deferred_split_shrinker is never called
from shrink_slab() for subordinate mem cgroups. It's called only
for root_mem_cgroup.

>> This is a mere clean up AFAICS.
>>
>>> Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>>>
>>>> ---
>>>>  mm/vmscan.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index ee4eecc..e7f10c4 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -422,7 +422,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
>>>>  {
>>>>  	down_write(&shrinker_rwsem);
>>>>  	list_add_tail(&shrinker->list, &shrinker_list);
>>>> -#ifdef CONFIG_MEMCG_KMEM
>>>> +#ifdef CONFIG_MEMCG
>>>>  	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>>>  		idr_replace(&shrinker_idr, shrinker, shrinker->id);
>>>>  #endif
>>>>
>>
>