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Subject[v7 PATCH 03/12] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation
Date
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.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Yang Shi <shy828301@gmail.com>
---
mm/vmscan.c | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 96b08c79f18d..e4ddaaaeffe2 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 free_shrinker_map_rcu(struct rcu_head *head)
{
@@ -200,8 +199,6 @@ static int 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);
@@ -249,7 +246,7 @@ 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);
@@ -260,7 +257,7 @@ 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;
}
@@ -275,9 +272,8 @@ 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;

memcg = mem_cgroup_iter(NULL, NULL, NULL);
do {
@@ -286,13 +282,13 @@ 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;
}

--
2.26.2
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