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Date: Sun, 26 Feb 2023 21:56:10 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Kirill Tkhai <tkhai@...ru>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, shakeelb@...gle.com,
mhocko@...nel.org, roman.gushchin@...ux.dev, muchun.song@...ux.dev,
david@...hat.com, shy828301@...il.com, dave@...olabs.net,
penguin-kernel@...ove.sakura.ne.jp, paulmck@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Sultan Alsawaf <sultan@...neltoast.com>
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
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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