Message-ID: <ee7852cc-e37e-5602-7729-5bb1500a1f2a@bytedance.com>
Date:   Thu, 23 Feb 2023 12:37:19 +0800
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Subject: Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless
Content-Language: en-US
To:     Kirill Tkhai <tkhai@ya.ru>
Cc:     sultan@kerneltoast.com, dave@stgolabs.net,
        penguin-kernel@I-love.SAKURA.ne.jp, paulmck@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        David Hildenbrand <david@redhat.com>,
        Yang Shi <shy828301@gmail.com>
References: <20230220091637.64865-1-zhengqi.arch@bytedance.com>
 <20230220091637.64865-3-zhengqi.arch@bytedance.com>
 <dc279728-4cd4-0453-1a28-fe076f254641@ya.ru>
 <39cf7fa7-5dd6-0424-23de-1c33e948a201@bytedance.com>
 <b9d344d0-9d27-034d-d59e-9e880318906b@bytedance.com>
 <23fde2e7-e9e2-2b0d-dfd8-1a654bc5503c@ya.ru>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <23fde2e7-e9e2-2b0d-dfd8-1a654bc5503c@ya.ru>
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On 2023/2/23 04:05, Kirill Tkhai wrote:
> On 22.02.2023 11:21, Qi Zheng wrote:
>>
>>
>> On 2023/2/22 16:16, Qi Zheng wrote:
>>> Hi Kirill,
>>>
>>> On 2023/2/22 05:43, Kirill Tkhai wrote:
>>>> On 20.02.2023 12:16, Qi Zheng wrote:
>>>>> Like global slab shrink, since commit 1cd0bd06093c<...>
>>>>>    static bool cgroup_reclaim(struct scan_control *sc)
>>>>> @@ -891,15 +905,14 @@ 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;
>>>>>        if (!mem_cgroup_online(memcg))
>>>>>            return 0;
>>>>> -    if (!down_read_trylock(&shrinker_rwsem))
>>>>> -        return 0;
>>>>> -
>>>>> -    info = shrinker_info_protected(memcg, nid);
>>>>> +    srcu_idx = srcu_read_lock(&shrinker_srcu);
>>>>> +    info = shrinker_info_srcu(memcg, nid);
>>>>>        if (unlikely(!info))
>>>>>            goto unlock;
>>>>
>>>> There is shrinker_nr_max dereference under this hunk. It's not in the patch:
>>>>
>>>>           for_each_set_bit(i, info->map, shrinker_nr_max) {
>>>>
>>>> Since shrinker_nr_max may grow in parallel, this leads to access beyond allocated memory :(
>>>
>>> Oh, indeed.
>>>
>>>>
>>>> It looks like we should save size of info->map as a new member of struct shrinker_info.
>>>
>>> Agree, then we only traverse info->map_size each time. Like below:
>>>
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index b6eda2ab205d..f1b5d2803007 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -97,6 +97,7 @@ struct shrinker_info {
>>>           struct rcu_head rcu;
>>>           atomic_long_t *nr_deferred;
>>>           unsigned long *map;
>>> +       int map_size;
> 
> Sure, like this. The only thing (after rethinking) I want to say is that despite "size" was
> may suggestion, now it makes me think that name "size" is about size in bytes.
> 
> Possible, something like map_nr_max would be better here.

Agree. Will change to it.

> 
>>>    };
>>>
>>>    struct lruvec_stats_percpu {
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index f94bfe540675..dd07eb107915 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -239,14 +239,20 @@ static void free_shrinker_info_rcu(struct rcu_head *head)
>>>           kvfree(container_of(head, struct shrinker_info, rcu));
>>>    }
>>>
>>> -static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>> -                                   int map_size, int defer_size,
>>> -                                   int old_map_size, int old_defer_size)
>>> +static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_nr_max,
>>> +                                   int old_nr_max)
>>>    {
>>> +       int map_size, defer_size, old_map_size, old_defer_size;
>>>           struct shrinker_info *new, *old;
>>>           struct mem_cgroup_per_node *pn;
>>>           int nid;
>>> -       int size = map_size + defer_size;
>>> +       int size;
>>> +
>>> +       map_size = shrinker_map_size(new_nr_max);
>>> +       defer_size = shrinker_defer_size(new_nr_max);
>>> +       old_map_size = shrinker_map_size(shrinker_nr_max);
>>> +       old_defer_size = shrinker_defer_size(shrinker_nr_max);
>>
>> Perhaps these should still be calculated outside the loop as before.
> 
> Yeah, for me it's also better.
>   
>>> +       size = map_size + defer_size;
>>>
>>>           for_each_node(nid) {
>>>                   pn = memcg->nodeinfo[nid];
>>> @@ -261,6 +267,7 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>>
>>>                   new->nr_deferred = (atomic_long_t *)(new + 1);
>>>                   new->map = (void *)new->nr_deferred + defer_size;
>>> +               new->map_size = new_nr_max;
>>>
>>>                   /* map: set all old bits, clear all new bits */
>>>                   memset(new->map, (int)0xff, old_map_size);
>>> @@ -310,6 +317,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>                   }
>>>                   info->nr_deferred = (atomic_long_t *)(info + 1);
>>>                   info->map = (void *)info->nr_deferred + defer_size;
>>> +               info->map_size = shrinker_nr_max;
>>>                   rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>           }
>>>           mutex_unlock(&shrinker_mutex);
>>> @@ -327,8 +335,6 @@ static int expand_shrinker_info(int new_id)
>>>    {
>>>           int ret = 0;
>>>           int new_nr_max = new_id + 1;
>>> -       int map_size, defer_size = 0;
>>> -       int old_map_size, old_defer_size = 0;
>>>           struct mem_cgroup *memcg;
>>>
>>>           if (!need_expand(new_nr_max))
>>> @@ -339,15 +345,9 @@ static int expand_shrinker_info(int new_id)
>>>
>>>           lockdep_assert_held(&shrinker_mutex);
>>>
>>> -       map_size = shrinker_map_size(new_nr_max);
>>> -       defer_size = shrinker_defer_size(new_nr_max);
>>> -       old_map_size = shrinker_map_size(shrinker_nr_max);
>>> -       old_defer_size = shrinker_defer_size(shrinker_nr_max);
>>> -
>>>           memcg = mem_cgroup_iter(NULL, NULL, NULL);
>>>           do {
>>> -               ret = expand_one_shrinker_info(memcg, map_size, defer_size,
>>> -                                              old_map_size, old_defer_size);
>>> +               ret = expand_one_shrinker_info(memcg, new_nr_max, shrinker_nr_max);
>>>                   if (ret) {
>>>                           mem_cgroup_iter_break(NULL, memcg);
>>>                           goto out;
>>> @@ -912,7 +912,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>           if (unlikely(!info))
>>>                   goto unlock;
>>>
>>> -       for_each_set_bit(i, info->map, shrinker_nr_max) {
>>> +       for_each_set_bit(i, info->map, info->map_size) {
>>>                   struct shrink_control sc = {
>>>                           .gfp_mask = gfp_mask,
>>>                           .nid = nid,
>>>
>>> I will send the v2.
>>>
>>> Thanks,
>>> Qi
>>>
>>>>
>>>>> @@ -949,14 +962,9 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>>>                    set_shrinker_bit(memcg, nid, i);
>>>>>            }
>>>>>            freed += ret;
>>>>> -
>>>>> -        if (rwsem_is_contended(&shrinker_rwsem)) {
>>>>> -            freed = freed ? : 1;
>>>>> -            break;
>>>>> -        }
>>>>>        }
>>>>>    unlock:
>>>>> -    up_read(&shrinker_rwsem);
>>>>> +    srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>>>        return freed;
>>>>>    }
>>>>>    #else /* CONFIG_MEMCG */
>>>>
>>>
>>
> 

-- 
Thanks,
Qi