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Message-ID: <2ba86f45-f0a5-3a85-4aa6-f8beb50491b3@bytedance.com>
Date: Sun, 26 Feb 2023 00:37:34 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Kirill Tkhai <tkhai@...ru>, Sultan Alsawaf <sultan@...neltoast.com>
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
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
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?
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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