Re: [PATCH 4/5] drm/ttm: introduce pool_shrink_rwsem

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Christian,

On 2023/8/16 17:14, Christian König wrote:
> Am 16.08.23 um 10:34 schrieb Qi Zheng:
>> Currently, the synchronize_shrinkers() is only used by TTM pool. It only
>> requires that no shrinkers run in parallel.
>>
>> After we use RCU+refcount method to implement the lockless slab shrink,
>> we can not use shrinker_rwsem or synchronize_rcu() to guarantee that all
>> shrinker invocations have seen an update before freeing memory.
>>
>> So we introduce a new pool_shrink_rwsem to implement a private
>> synchronize_shrinkers(), so as to achieve the same purpose.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_pool.c | 15 +++++++++++++++
>>   include/linux/shrinker.h       |  1 -
>>   mm/shrinker.c                  | 15 ---------------
>>   3 files changed, 15 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c 
>> b/drivers/gpu/drm/ttm/ttm_pool.c
>> index cddb9151d20f..713b1c0a70e1 100644
>> --- a/drivers/gpu/drm/ttm/ttm_pool.c
>> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
>> @@ -74,6 +74,7 @@ static struct ttm_pool_type 
>> global_dma32_uncached[MAX_ORDER + 1];
>>   static spinlock_t shrinker_lock;
>>   static struct list_head shrinker_list;
>>   static struct shrinker mm_shrinker;
>> +static DECLARE_RWSEM(pool_shrink_rwsem);
>>   /* Allocate pages of size 1 << order with the given gfp_flags */
>>   static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t 
>> gfp_flags,
>> @@ -317,6 +318,7 @@ static unsigned int ttm_pool_shrink(void)
>>       unsigned int num_pages;
>>       struct page *p;
>> +    down_read(&pool_shrink_rwsem);
>>       spin_lock(&shrinker_lock);
>>       pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
>>       list_move_tail(&pt->shrinker_list, &shrinker_list);
>> @@ -329,6 +331,7 @@ static unsigned int ttm_pool_shrink(void)
>>       } else {
>>           num_pages = 0;
>>       }
>> +    up_read(&pool_shrink_rwsem);
>>       return num_pages;
>>   }
>> @@ -572,6 +575,18 @@ void ttm_pool_init(struct ttm_pool *pool, struct 
>> device *dev,
>>   }
>>   EXPORT_SYMBOL(ttm_pool_init);
>> +/**
>> + * synchronize_shrinkers - Wait for all running shrinkers to complete.
>> + *
>> + * This is useful to guarantee that all shrinker invocations have 
>> seen an
>> + * update, before freeing memory, similar to rcu.
>> + */
>> +static void synchronize_shrinkers(void)
> 
> Please rename that function to ttm_pool_synchronize_shrinkers().

OK, will do.

> 
> With that done feel free to add Reviewed-by: Christian König 
> <christian.koenig@amd.com>
> 

Thanks,
Qi

> Regards,
> Christian.
> 
>> +{
>> +    down_write(&pool_shrink_rwsem);
>> +    up_write(&pool_shrink_rwsem);
>> +}
>> +
>>   /**
>>    * ttm_pool_fini - Cleanup a pool
>>    *
>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>> index 8dc15aa37410..6b5843c3b827 100644
>> --- a/include/linux/shrinker.h
>> +++ b/include/linux/shrinker.h
>> @@ -103,7 +103,6 @@ extern int __printf(2, 3) register_shrinker(struct 
>> shrinker *shrinker,
>>                           const char *fmt, ...);
>>   extern void unregister_shrinker(struct shrinker *shrinker);
>>   extern void free_prealloced_shrinker(struct shrinker *shrinker);
>> -extern void synchronize_shrinkers(void);
>>   #ifdef CONFIG_SHRINKER_DEBUG
>>   extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker 
>> *shrinker,
>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>> index 043c87ccfab4..a16cd448b924 100644
>> --- a/mm/shrinker.c
>> +++ b/mm/shrinker.c
>> @@ -692,18 +692,3 @@ void unregister_shrinker(struct shrinker *shrinker)
>>       shrinker->nr_deferred = NULL;
>>   }
>>   EXPORT_SYMBOL(unregister_shrinker);
>> -
>> -/**
>> - * synchronize_shrinkers - Wait for all running shrinkers to complete.
>> - *
>> - * This is equivalent to calling unregister_shrink() and 
>> register_shrinker(),
>> - * but atomically and with less overhead. This is useful to guarantee 
>> that all
>> - * shrinker invocations have seen an update, before freeing memory, 
>> similar to
>> - * rcu.
>> - */
>> -void synchronize_shrinkers(void)
>> -{
>> -    down_write(&shrinker_rwsem);
>> -    up_write(&shrinker_rwsem);
>> -}
>> -EXPORT_SYMBOL(synchronize_shrinkers);
>