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SubjectRe: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
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On 2023/2/26 05:28, Kirill Tkhai wrote:
> On 25.02.2023 19:37, Qi Zheng wrote:
>>
>>
>> On 2023/2/26 00:17, Kirill Tkhai wrote:
>>> On 25.02.2023 18:57, Qi Zheng wrote:
>>>>
>> <...>
>>>> How about this?
>>>>>>
>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>>> index ffddbd204259..9d8c53075298 100644
>>>>>> --- a/mm/vmscan.c
>>>>>> +++ b/mm/vmscan.c
>>>>>> @@ -1012,7 +1012,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>                                    int priority)
>>>>>>    {
>>>>>>           unsigned long ret, freed = 0;
>>>>>> -       struct shrinker *shrinker;
>>>>>> +       struct shrinker *shrinker = NULL;
>>>>>>           int srcu_idx, generation;
>>>>>>
>>>>>>           /*
>>>>>> @@ -1025,11 +1025,15 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>           if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>>>>>                   return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>>>>>
>>>>>> +again:
>>>>>>           srcu_idx = srcu_read_lock(&shrinker_srcu);
>>>>>>
>>>>>>           generation = atomic_read(&shrinker_srcu_generation);
>>>>>> -       list_for_each_entry_srcu(shrinker, &shrinker_list, list,
>>>>>> -                                srcu_read_lock_held(&shrinker_srcu)) {
>>>>>> +       if (!shrinker)
>>>>>> +               shrinker = list_entry_rcu(shrinker_list.next, struct shrinker, list);
>>>>>> +       else
>>>>>> +               shrinker = list_entry_rcu(shrinker->list.next, struct shrinker, list);
>>>>>> +       list_for_each_entry_from_rcu(shrinker, &shrinker_list, list) {
>>>>>>                   struct shrink_control sc = {
>>>>>>                           .gfp_mask = gfp_mask,
>>>>>>                           .nid = nid,
>>>>>> @@ -1042,8 +1046,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>                   freed += ret;
>>>>>>
>>>>>>                   if (atomic_read(&shrinker_srcu_generation) != generation) {
>>>>>> -                       freed = freed ? : 1;
>>>>>> -                       break;
>>>>>> +                       srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>
>>> After SRCU in unlocked we can't believe @shrinker anymore. So, above list_entry_rcu(shrinker->list.next)
>>> dereferences some random memory.
>>
>> Indeed.
>>
>>>
>>>>>> +                       cond_resched();
>>>>>> +                       goto again;
>>>>>>                   }
>>>>>>           }
>>>>>>
>>>>>>>
>>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>>>> index 27ef9946ae8a..0b197bba1257 100644
>>>>>>> --- a/mm/vmscan.c
>>>>>>> +++ b/mm/vmscan.c
>>>>>>> @@ -204,6 +204,7 @@ static void set_task_reclaim_state(struct task_struct *task,
>>>>>>>     LIST_HEAD(shrinker_list);
>>>>>>>     DEFINE_MUTEX(shrinker_mutex);
>>>>>>>     DEFINE_SRCU(shrinker_srcu);
>>>>>>> +static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
>>>>>>>       #ifdef CONFIG_MEMCG
>>>>>>>     static int shrinker_nr_max;
>>>>>>> @@ -782,6 +783,7 @@ void unregister_shrinker(struct shrinker *shrinker)
>>>>>>>         debugfs_entry = shrinker_debugfs_remove(shrinker);
>>>>>>>         mutex_unlock(&shrinker_mutex);
>>>>>>>     +    atomic_inc(&shrinker_srcu_generation);
>>>>>>>         synchronize_srcu(&shrinker_srcu);
>>>>>>>           debugfs_remove_recursive(debugfs_entry);
>>>>>>> @@ -799,6 +801,7 @@ EXPORT_SYMBOL(unregister_shrinker);
>>>>>>>      */
>>>>>>>     void synchronize_shrinkers(void)
>>>>>>>     {
>>>>>>> +    atomic_inc(&shrinker_srcu_generation);
>>>>>>>         synchronize_srcu(&shrinker_srcu);
>>>>>>>     }
>>>>>>>     EXPORT_SYMBOL(synchronize_shrinkers);
>>>>>>> @@ -908,18 +911,19 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>>>>>     {
>>>>>>>         struct shrinker_info *info;
>>>>>>>         unsigned long ret, freed = 0;
>>>>>>> -    int srcu_idx;
>>>>>>> -    int i;
>>>>>>> +    int srcu_idx, generation;
>>>>>>> +    int i = 0;
>>>>>>>           if (!mem_cgroup_online(memcg))
>>>>>>>             return 0;
>>>>>>> -
>>>>>>> +again:
>>>>>>>         srcu_idx = srcu_read_lock(&shrinker_srcu);
>>>>>>>         info = shrinker_info_srcu(memcg, nid);
>>>>>>>         if (unlikely(!info))
>>>>>>>             goto unlock;
>>>>>>>     -    for_each_set_bit(i, info->map, info->map_nr_max) {
>>>>>>> +    generation = atomic_read(&shrinker_srcu_generation);
>>>>>>> +    for_each_set_bit_from(i, info->map, info->map_nr_max) {
>>>>>>>             struct shrink_control sc = {
>>>>>>>                 .gfp_mask = gfp_mask,
>>>>>>>                 .nid = nid,
>>>>>>> @@ -965,6 +969,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>>>>>                     set_shrinker_bit(memcg, nid, i);
>>>>>>>             }
>>>>>>>             freed += ret;
>>>>>>> +
>>>>>>> +        if (atomic_read(&shrinker_srcu_generation) != generation) {
>>>>>>> +            srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>>>>
>>>>>> Maybe we can add the following code here, so as to avoid repeating the
>>>>>> current id and avoid triggering softlockup:
>>>>>>
>>>>>>               i++;
>>>
>>> This is OK.
>>>
>>>>>>               cond_resched();
>>>
>>> Possible, existing cond_resched() in do_shrink_slab() is enough.
>>
>> Yeah.
>>
>> I will add this patch in the next version. May I mark you as the author
>> of this patch?
>
> I think, yes

Thanks. :)

Qi

>
>>>
>>>> And this. :)
>>>>
>>>> Thanks,
>>>> Qi
>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Qi
>>>>>>
>>>>>>> +            goto again;
>>>>>>> +        }
>>>>>>>         }
>>>>>>>     unlock:
>>>>>>>         srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>>>>> @@ -1004,7 +1013,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>>     {
>>>>>>>         unsigned long ret, freed = 0;
>>>>>>>         struct shrinker *shrinker;
>>>>>>> -    int srcu_idx;
>>>>>>> +    int srcu_idx, generation;
>>>>>>>           /*
>>>>>>>          * The root memcg might be allocated even though memcg is disabled
>>>>>>> @@ -1017,6 +1026,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>>             return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>>>>>>           srcu_idx = srcu_read_lock(&shrinker_srcu);
>>>>>>> +    generation = atomic_read(&shrinker_srcu_generation);
>>>>>>>           list_for_each_entry_srcu(shrinker, &shrinker_list, list,
>>>>>>>                      srcu_read_lock_held(&shrinker_srcu)) {
>>>>>>> @@ -1030,6 +1040,11 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>>>>>             if (ret == SHRINK_EMPTY)
>>>>>>>                 ret = 0;
>>>>>>>             freed += ret;
>>>>>>> +
>>>>>>> +        if (atomic_read(&shrinker_srcu_generation) != generation) {
>>>>>>> +            freed = freed ? : 1;
>>>>>>> +            break;
>>>>>>> +        }
>>>>>>>         }
>>>>>>>           srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

--
Thanks,
Qi

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