From: Yang Shi <shy828301@xxxxxxxxx>
Subject: 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@xxxxxxxxx
Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 18 ++++++++----------
1 file changed, 8 insertions(+), 10 deletions(-)
--- a/mm/vmscan.c~mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation
+++ a/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)
{
@@ -201,8 +200,6 @@ static int expand_one_shrinker_map(struc
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_cgrou
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_cgrou
}
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_
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_
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;
}
_
