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Message-ID: <23fde2e7-e9e2-2b0d-dfd8-1a654bc5503c@ya.ru>
Date: Wed, 22 Feb 2023 23:05:43 +0300
From: Kirill Tkhai <tkhai@...ru>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: sultan@...neltoast.com, dave@...olabs.net,
penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
Shakeel Butt <shakeelb@...gle.com>,
Michal Hocko <mhocko@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Muchun Song <muchun.song@...ux.dev>,
David Hildenbrand <david@...hat.com>,
Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless
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.
>> };
>>
>> 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 */
>>>
>>
>
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