On 22.02.2023 10:32, Qi Zheng wrote:
>
>
> On 2023/2/22 05:28, Kirill Tkhai wrote:
>> On 20.02.2023 12:16, Qi Zheng wrote:
> <...>
>>> void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>> {
>>> - int i, nid;
>>> + int i, nid, srcu_idx;
>>> long nr;
>>> struct mem_cgroup *parent;
>>> struct shrinker_info *child_info, *parent_info;
>>> @@ -429,16 +443,16 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>> parent = root_mem_cgroup;
>>> /* Prevent from concurrent shrinker_info expand */
>>> - down_read(&shrinker_rwsem);
>>> + srcu_idx = srcu_read_lock(&shrinker_srcu);
>>
>> Don't we still have to be protected against parallel expand_one_shrinker_info()?
>>
>> It looks like parent->nodeinfo[nid]->shrinker_info pointer may be changed in expand*
>> right after we've dereferenced it here.
>
> Hi Kirill,
>
> Oh, indeed. We may wrongly reparent the child's nr_deferred to the old
> parent's nr_deferred (it is about to be freed by call_srcu).
>
> The reparent_shrinker_deferred() will only be called on the offline
> path (not a hotspot path), so we may be able to use shrinker_mutex
> introduced later for protection. What do you think?
It looks good for me. One more thing I'd analyzed is whether we want to have
is two reparent_shrinker_deferred() are executing in parallel.
Possible, we should leave rwsem there as it was used before..
>>
>>> for_each_node(nid) {
>>> - child_info = shrinker_info_protected(memcg, nid);
>>> - parent_info = shrinker_info_protected(parent, nid);
>>> + child_info = shrinker_info_srcu(memcg, nid);
>>> + parent_info = shrinker_info_srcu(parent, nid);
>>> for (i = 0; i < shrinker_nr_max; i++) {
>>> nr = atomic_long_read(&child_info->nr_deferred[i]);
>>> atomic_long_add(nr, &parent_info->nr_deferred[i]);
>>> }
>>> }
>>> - up_read(&shrinker_rwsem);
>>> + srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>> }
>>> 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;
>>> @@ -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 */
>>
>
