+ mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch added to -mm tree

+ mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation
has been added to the -mm tree.  Its filename is
     mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch

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------------------------------------------------------
From: Yang Shi <shy828301@gmail.com>
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 <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>
---

 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;
 }
 
_

Patches currently in -mm which might be from shy828301@gmail.com are

mm-vmscan-use-nid-from-shrink_control-for-tracepoint.patch
mm-vmscan-consolidate-shrinker_maps-handling-code.patch
mm-vmscan-use-shrinker_rwsem-to-protect-shrinker_maps-allocation.patch
mm-vmscan-remove-memcg_shrinker_map_size.patch
mm-vmscan-use-kvfree_rcu-instead-of-call_rcu.patch
mm-memcontrol-rename-shrinker_map-to-shrinker_info.patch
mm-vmscan-add-shrinker_info_protected-helper.patch
mm-vmscan-use-a-new-flag-to-indicate-shrinker-is-registered.patch
mm-vmscan-add-per-memcg-shrinker-nr_deferred.patch
mm-vmscan-use-per-memcg-nr_deferred-of-shrinker.patch
mm-vmscan-dont-need-allocate-shrinker-nr_deferred-for-memcg-aware-shrinkers.patch
mm-memcontrol-reparent-nr_deferred-when-memcg-offline.patch
mm-vmscan-shrink-deferred-objects-proportional-to-priority.patch