Re: [v3 PATCH 05/11] mm: vmscan: use a new flag to indicate shrinker is registered

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 11.01.2021 21:17, Yang Shi wrote:
> On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>> On 06.01.2021 01:58, Yang Shi wrote:
>>> Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
>>> This approach is fine with nr_deferred at the shrinker level, but the following
>>> patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
>>> shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
>>> from unregistering correctly.
>>>
>>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>>> ---
>>>  include/linux/shrinker.h |  7 ++++---
>>>  mm/vmscan.c              | 13 +++++++++----
>>>  2 files changed, 13 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>>> index 0f80123650e2..1eac79ce57d4 100644
>>> --- a/include/linux/shrinker.h
>>> +++ b/include/linux/shrinker.h
>>> @@ -79,13 +79,14 @@ struct shrinker {
>>>  #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
>>>
>>>  /* Flags */
>>> -#define SHRINKER_NUMA_AWARE  (1 << 0)
>>> -#define SHRINKER_MEMCG_AWARE (1 << 1)
>>> +#define SHRINKER_REGISTERED  (1 << 0)
>>> +#define SHRINKER_NUMA_AWARE  (1 << 1)
>>> +#define SHRINKER_MEMCG_AWARE (1 << 2)
>>>  /*
>>>   * It just makes sense when the shrinker is also MEMCG_AWARE for now,
>>>   * non-MEMCG_AWARE shrinker should not have this flag set.
>>>   */
>>> -#define SHRINKER_NONSLAB     (1 << 2)
>>> +#define SHRINKER_NONSLAB     (1 << 3)
>>>
>>>  extern int prealloc_shrinker(struct shrinker *shrinker);
>>>  extern void register_shrinker_prepared(struct shrinker *shrinker);
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 8da765a85569..9761c7c27412 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -494,6 +494,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
>>>       if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>>               idr_replace(&shrinker_idr, shrinker, shrinker->id);
>>>  #endif
>>> +     shrinker->flags |= SHRINKER_REGISTERED;
>>
>> In case of we introduce this new flag, we should kill old flag SHRINKER_REGISTERING,
>> which are not needed anymore (we should you the new flag instead of that).
> 
> The only think that I'm confused with is the check in
> shrink_slab_memcg, it does:
> 
> shrinker = idr_find(&shrinker_idr, i);
> if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
> 
> When allocating idr, the shrinker is associated with
> SHRINKER_REGISTERING. But, shrink_slab_memcg does acquire read
> shrinker_rwsem, and idr_alloc is called with holding write
> shrinker_rwsem, so I'm supposed shrink_slab_memcg should never see
> shrinker is registering.

After prealloc_shrinker() shrinker is visible for shrink_slab_memcg().
This is the moment shrink_slab_memcg() sees SHRINKER_REGISTERED.

> If so it seems easy to remove
> SHRINKER_REGISTERING.
> 
> We just need change that check to:
> !shrinker || !(shrinker->flags & SHRINKER_REGISTERED)
> 
>>>       up_write(&shrinker_rwsem);
>>>  }
>>>
>>> @@ -513,13 +514,17 @@ EXPORT_SYMBOL(register_shrinker);
>>>   */
>>>  void unregister_shrinker(struct shrinker *shrinker)
>>>  {
>>> -     if (!shrinker->nr_deferred)
>>> -             return;
>>> -     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>> -             unregister_memcg_shrinker(shrinker);
>>>       down_write(&shrinker_rwsem);
>>
>> I do not think there are some users which registration may race with unregistration.
>> So, I think we should check SHRINKER_REGISTERED unlocked similar to we used to check
>> shrinker->nr_deferred unlocked.
> 
> Yes, I agree.
> 
>>
>>> +     if (!(shrinker->flags & SHRINKER_REGISTERED)) {
>>> +             up_write(&shrinker_rwsem);
>>> +             return;
>>> +     }
>>>       list_del(&shrinker->list);
>>> +     shrinker->flags &= ~SHRINKER_REGISTERED;
>>>       up_write(&shrinker_rwsem);
>>> +
>>> +     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>> +             unregister_memcg_shrinker(shrinker);
>>>       kfree(shrinker->nr_deferred);
>>>       shrinker->nr_deferred = NULL;
>>>  }
>>>
>>
>>