On 11.01.2021 21:17, Yang Shi wrote:
> On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> 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@xxxxxxxxx>
>>> ---
>>> 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;
>>> }
>>>
>>
>>
