Re: [PATCH v2 1/7] mm: vmscan: add a map_nr_max field to shrinker_info

[Kirill Tkhai <tkhai@ya.ru>]

Hi Qi,

On 25.02.2023 11:18, Qi Zheng wrote:
> 
> 
> On 2023/2/23 21:27, Qi Zheng wrote:
>> To prepare for the subsequent lockless memcg slab shrink,
>> add a map_nr_max field to struct shrinker_info to records
>> its own real shrinker_nr_max.
>>
>> No functional changes.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> I missed Suggested-by here, hi Kirill, can I add it?
> 
> Suggested-by: Kirill Tkhai <tkhai@ya.ru>

Yes, feel free to add this tag.

There is a comment below.

>> ---
>>   include/linux/memcontrol.h |  1 +
>>   mm/vmscan.c                | 29 ++++++++++++++++++-----------
>>   2 files changed, 19 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index b6eda2ab205d..aa69ea98e2d8 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_nr_max;
>>   };
>>     struct lruvec_stats_percpu {
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 9c1c5e8b24b8..9f895ca6216c 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -224,9 +224,16 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>>                        lockdep_is_held(&shrinker_rwsem));
>>   }
>>   +static inline bool need_expand(int new_nr_max, int old_nr_max)
>> +{
>> +    return round_up(new_nr_max, BITS_PER_LONG) >
>> +           round_up(old_nr_max, BITS_PER_LONG);
>> +}
>> +
>>   static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>                       int map_size, int defer_size,
>> -                    int old_map_size, int old_defer_size)
>> +                    int old_map_size, int old_defer_size,
>> +                    int new_nr_max)
>>   {
>>       struct shrinker_info *new, *old;
>>       struct mem_cgroup_per_node *pn;
>> @@ -240,12 +247,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>           if (!old)
>>               return 0;
>>   +        if (!need_expand(new_nr_max, old->map_nr_max))
>> +            return 0;
>> +
>>           new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
>>           if (!new)
>>               return -ENOMEM;
>>             new->nr_deferred = (atomic_long_t *)(new + 1);
>>           new->map = (void *)new->nr_deferred + defer_size;
>> +        new->map_nr_max = new_nr_max;
>>             /* map: set all old bits, clear all new bits */
>>           memset(new->map, (int)0xff, old_map_size);
>> @@ -295,6 +306,7 @@ 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_nr_max = shrinker_nr_max;
>>           rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>       }
>>       up_write(&shrinker_rwsem);
>> @@ -302,12 +314,6 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>       return ret;
>>   }
>>   -static inline bool need_expand(int nr_max)
>> -{
>> -    return round_up(nr_max, BITS_PER_LONG) >
>> -           round_up(shrinker_nr_max, BITS_PER_LONG);
>> -}
>> -
>>   static int expand_shrinker_info(int new_id)
>>   {
>>       int ret = 0;
>> @@ -316,7 +322,7 @@ static int expand_shrinker_info(int new_id)
>>       int old_map_size, old_defer_size = 0;
>>       struct mem_cgroup *memcg;
>>   -    if (!need_expand(new_nr_max))
>> +    if (!need_expand(new_nr_max, shrinker_nr_max))
>>           goto out;
>>         if (!root_mem_cgroup)
>> @@ -332,7 +338,8 @@ static int expand_shrinker_info(int new_id)
>>       memcg = mem_cgroup_iter(NULL, NULL, NULL);
>>       do {
>>           ret = expand_one_shrinker_info(memcg, map_size, defer_size,
>> -                           old_map_size, old_defer_size);
>> +                           old_map_size, old_defer_size,
>> +                           new_nr_max);
>>           if (ret) {
>>               mem_cgroup_iter_break(NULL, memcg);
>>               goto out;
>> @@ -432,7 +439,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>       for_each_node(nid) {
>>           child_info = shrinker_info_protected(memcg, nid);
>>           parent_info = shrinker_info_protected(parent, nid);
>> -        for (i = 0; i < shrinker_nr_max; i++) {
>> +        for (i = 0; i < child_info->map_nr_max; i++) {
>>               nr = atomic_long_read(&child_info->nr_deferred[i]);
>>               atomic_long_add(nr, &parent_info->nr_deferred[i]);
>>           }
>> @@ -899,7 +906,7 @@ 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_nr_max) {
>>           struct shrink_control sc = {
>>               .gfp_mask = gfp_mask,
>>               .nid = nid,

The patch as whole thing won't work as expected. It won't ever call shrinker with ids from [round_down(shrinker_nr_max, sizeof(unsigned long)) + 1, shrinker_nr_max - 1]

Just replay the sequence we add new shrinkers:

1)We add shrinker #0:
   shrinker_nr_max = 0;

   prealloc_memcg_shrinker()
      id = 0;
      expand_shrinker_info(0)
        new_nr_max = 1;
        expand_one_shrinker_info(new_nr_max = 1)
          new->map_nr_max = 1;
        shrinker_nr_max = 1;

2)We add shrinker #1:
   prealloc_memcg_shrinker()
     id = 1;
     expand_shrinker_info(1)
       new_nr_max = 2;
       need_expand(2, 1) => false => ignore expand
       shrinker_nr_max = 2;

3)Then we call shrinker:
  shrink_slab_memcg()
    for_each_set_bit(i, info->map, 1/* info->map_nr_max */ ) {
    } => ignore shrinker #1

I'd fixed this patch by something like the below:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f895ca6216c..bb617a3871f1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -224,12 +224,6 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
 					 lockdep_is_held(&shrinker_rwsem));
 }
 
-static inline bool need_expand(int new_nr_max, int old_nr_max)
-{
-	return round_up(new_nr_max, BITS_PER_LONG) >
-	       round_up(old_nr_max, BITS_PER_LONG);
-}
-
 static int expand_one_shrinker_info(struct mem_cgroup *memcg,
 				    int map_size, int defer_size,
 				    int old_map_size, int old_defer_size,
@@ -247,9 +241,6 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
 		if (!old)
 			return 0;
 
-		if (!need_expand(new_nr_max, old->map_nr_max))
-			return 0;
-
 		new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
 		if (!new)
 			return -ENOMEM;
@@ -317,14 +308,11 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
 static int expand_shrinker_info(int new_id)
 {
 	int ret = 0;
-	int new_nr_max = new_id + 1;
+	int new_nr_max = round_up(new_id + 1, BITS_PER_LONG);
 	int map_size, defer_size = 0;
 	int old_map_size, old_defer_size = 0;
 	struct mem_cgroup *memcg;
 
-	if (!need_expand(new_nr_max, shrinker_nr_max))
-		goto out;
-
 	if (!root_mem_cgroup)
 		goto out;
 
@@ -359,9 +347,11 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
 
 		rcu_read_lock();
 		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
-		/* Pairs with smp mb in shrink_slab() */
-		smp_mb__before_atomic();
-		set_bit(shrinker_id, info->map);
+		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
+			/* Pairs with smp mb in shrink_slab() */
+			smp_mb__before_atomic();
+			set_bit(shrinker_id, info->map);
+		}
 		rcu_read_unlock();
 	}
 }

(I also added a new check into set_shrinker_bit() for safety).

Kirill