[patch 064/143] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2021-05-05 01:36:17
Also in:
mm-commits
From: Yang Shi <redacted>
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@gmail.com
Signed-off-by: Yang Shi <redacted>
Acked-by: Vlastimil Babka <redacted>
Acked-by: Kirill Tkhai <redacted>
Acked-by: Roman Gushchin <redacted>
Reviewed-by: Shakeel Butt <redacted>
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>
---
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; }
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