Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless

[Qi Zheng <zhengqi.arch@bytedance.com>]

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;
  };

  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);
+       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