On 06.01.2021 01:58, Yang Shi wrote: > Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem > exclusively, the read side can be protected by holding read lock, so it sounds > superfluous to have a dedicated mutex. This should not exacerbate the contention > to shrinker_rwsem since just one read side critical section is added. > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> > --- > mm/vmscan.c | 16 ++++++---------- > 1 file changed, 6 insertions(+), 10 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 9db7b4d6d0ae..ddb9f972f856 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem); > #ifdef CONFIG_MEMCG > > static int memcg_shrinker_map_size; > -static DEFINE_MUTEX(memcg_shrinker_map_mutex); > > static void memcg_free_shrinker_map_rcu(struct rcu_head *head) > { > @@ -200,8 +199,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg, > struct memcg_shrinker_map *new, *old; > int nid; > > - lockdep_assert_held(&memcg_shrinker_map_mutex); > - > for_each_node(nid) { > old = rcu_dereference_protected( > mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true); > @@ -250,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg) > if (mem_cgroup_is_root(memcg)) > return 0; > > - mutex_lock(&memcg_shrinker_map_mutex); > + down_read(&shrinker_rwsem); > size = memcg_shrinker_map_size; > for_each_node(nid) { > map = kvzalloc(sizeof(*map) + size, GFP_KERNEL); > @@ -261,7 +258,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg) > } > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map); Here we do STORE operation, and since we want the assignment is visible for shrink_slab_memcg() under down_read(), we have to use down_write() in memcg_alloc_shrinker_maps(). > } > - mutex_unlock(&memcg_shrinker_map_mutex); > + up_read(&shrinker_rwsem); > > return ret; > } > @@ -276,9 +273,8 @@ static int memcg_expand_shrinker_maps(int new_id) > if (size <= old_size) > return 0; > > - mutex_lock(&memcg_shrinker_map_mutex); > if (!root_mem_cgroup) > - goto unlock; > + goto out; > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > do { > @@ -287,13 +283,13 @@ static int memcg_expand_shrinker_maps(int new_id) > ret = memcg_expand_one_shrinker_map(memcg, size, old_size); > if (ret) { > mem_cgroup_iter_break(NULL, memcg); > - goto unlock; > + goto out; > } > } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); > -unlock: > +out: > if (!ret) > memcg_shrinker_map_size = size; > - mutex_unlock(&memcg_shrinker_map_mutex); > + > return ret; > } > >