Commit d27cf2aa authored by Yang Shi's avatar Yang Shi Committed by Linus Torvalds
Browse files

mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation 

Since memcg_shrinker_map_size just can be changed under holding
shrinker_rwsem exclusively, the read side can be protected by holding read
lock, so it sounds superfluous to have a dedicated mutex.

Kirill Tkhai suggested use write lock since:

  * We want the assignment to shrinker_maps is visible for shrink_slab_memcg().
  * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but
    in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing
    is not actually protected.
  * READ lock makes alloc_shrinker_info() racy against memory allocation fail.
    alloc_shrinker_info()->free_shrinker_info() may free memory right after
    shrink_slab_memcg() dereferenced it. You may say
    shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure,
    but this is not the thing we want to remember in the future, since this
    spreads modularity.

And a test with heavy paging workload didn't show write lock makes things worse.

Link: https://lkml.kernel.org/r/20210311190845.9708-4-shy828301@gmail.com


Signed-off-by: default avatarYang Shi <shy828301@gmail.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Acked-by: default avatarKirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: default avatarRoman Gushchin <guro@fb.com>
Reviewed-by: default avatarShakeel Butt <shakeelb@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2bfd3637

mm/vmscan.c

+8 −10
Original line number Diff line number Diff line
@@ -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 free_shrinker_map_rcu(struct rcu_head *head)

{
@@ -201,8 +200,6 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg, 
	struct mem_cgroup_per_node *pn;

	int nid;



	lockdep_assert_held(&memcg_shrinker_map_mutex);



	for_each_node(nid) {

		pn = memcg->nodeinfo[nid];

		old = rcu_dereference_protected(pn->shrinker_map, true);
@@ -250,7 +247,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) 
	if (mem_cgroup_is_root(memcg))

		return 0;



	mutex_lock(&memcg_shrinker_map_mutex);

	down_write(&shrinker_rwsem);

	size = memcg_shrinker_map_size;

	for_each_node(nid) {

		map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
@@ -261,7 +258,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) 
		}

		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);

	}

	mutex_unlock(&memcg_shrinker_map_mutex);

	up_write(&shrinker_rwsem);



	return ret;

}
@@ -276,9 +273,10 @@ static int 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;



	lockdep_assert_held(&shrinker_rwsem);



	memcg = mem_cgroup_iter(NULL, NULL, NULL);

	do {
@@ -287,13 +285,13 @@ static int expand_shrinker_maps(int new_id) 
		ret = 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;

}