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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: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Yang Shi 2021-05-04 18:36:14 -07:00 committed by Linus Torvalds
parent 2bfd36374e
commit d27cf2aa0d
1 changed files with 8 additions and 10 deletions
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18
mm/vmscan.c
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@ -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;
}