From: Yang Shi shy828301@gmail.com
mainline inclusion from mainline-v5.13-rc1 commit d27cf2aa0d26a221982d04757cc32db97833ec29 category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/I48N0H CVE: NA
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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 Signed-off-by: Chen Wandun chenwandun@huawei.com Reviewed-by: Tong Tiangen tongtiangen@huawei.com Signed-off-by: Zheng Zengkai zhengzengkai@huawei.com --- mm/vmscan.c | 18 ++++++++---------- 1 file changed, 8 insertions(+), 10 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c index a34d3cf4bca9..2e2043c0c50f 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -190,7 +190,6 @@ 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) { @@ -204,8 +203,6 @@ 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); @@ -253,7 +250,7 @@ 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); @@ -264,7 +261,7 @@ 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; } @@ -279,9 +276,10 @@ 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 { @@ -290,13 +288,13 @@ 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; }