mm: memcontrol: convert reclaim iterator to simple css refcounting

The memcg reclaim iterators use a complicated weak reference scheme to
prevent pinning cgroups indefinitely in the absence of memory pressure.

However, during the ongoing cgroup core rework, css lifetime has been
decoupled such that a pinned css no longer interferes with removal of
the user-visible cgroup, and all this complexity is now unnecessary.

[mhocko@suse.cz: ensure that the cached reference is always released]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -143,14 +143,8 @@ struct mem_cgroup_stat_cpu {
 	unsigned long targets[MEM_CGROUP_NTARGETS];
 };
 
-struct mem_cgroup_reclaim_iter {
-	/*
-	 * last scanned hierarchy member. Valid only if last_dead_count
-	 * matches memcg->dead_count of the hierarchy root group.
-	 */
-	struct mem_cgroup *last_visited;
-	int last_dead_count;
-
+struct reclaim_iter {
+	struct mem_cgroup *position;
 	/* scan generation, increased every round-trip */
 	unsigned int generation;
 };
@@ -162,7 +156,7 @@ struct mem_cgroup_per_zone {
 	struct lruvec		lruvec;
 	unsigned long		lru_size[NR_LRU_LISTS];
 
-	struct mem_cgroup_reclaim_iter reclaim_iter[DEF_PRIORITY + 1];
+	struct reclaim_iter	iter[DEF_PRIORITY + 1];
 
 	struct rb_node		tree_node;	/* RB tree node */
 	unsigned long		usage_in_excess;/* Set to the value by which */
@@ -346,7 +340,6 @@ struct mem_cgroup {
 	struct mem_cgroup_stat_cpu nocpu_base;
 	spinlock_t pcp_counter_lock;
 
-	atomic_t	dead_count;
 #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_INET)
 	struct cg_proto tcp_mem;
 #endif
@@ -1067,122 +1060,6 @@ static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 	return memcg;
 }
 
-/*
- * Returns a next (in a pre-order walk) alive memcg (with elevated css
- * ref. count) or NULL if the whole root's subtree has been visited.
- *
- * helper function to be used by mem_cgroup_iter
- */
-static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
-		struct mem_cgroup *last_visited)
-{
-	struct cgroup_subsys_state *prev_css, *next_css;
-
-	prev_css = last_visited ? &last_visited->css : NULL;
-skip_node:
-	next_css = css_next_descendant_pre(prev_css, &root->css);
-
-	/*
-	 * Even if we found a group we have to make sure it is
-	 * alive. css && !memcg means that the groups should be
-	 * skipped and we should continue the tree walk.
-	 * last_visited css is safe to use because it is
-	 * protected by css_get and the tree walk is rcu safe.
-	 *
-	 * We do not take a reference on the root of the tree walk
-	 * because we might race with the root removal when it would
-	 * be the only node in the iterated hierarchy and mem_cgroup_iter
-	 * would end up in an endless loop because it expects that at
-	 * least one valid node will be returned. Root cannot disappear
-	 * because caller of the iterator should hold it already so
-	 * skipping css reference should be safe.
-	 */
-	if (next_css) {
-		struct mem_cgroup *memcg = mem_cgroup_from_css(next_css);
-
-		if (next_css == &root->css)
-			return memcg;
-
-		if (css_tryget_online(next_css)) {
-			/*
-			 * Make sure the memcg is initialized:
-			 * mem_cgroup_css_online() orders the the
-			 * initialization against setting the flag.
-			 */
-			if (smp_load_acquire(&memcg->initialized))
-				return memcg;
-			css_put(next_css);
-		}
-
-		prev_css = next_css;
-		goto skip_node;
-	}
-
-	return NULL;
-}
-
-static void mem_cgroup_iter_invalidate(struct mem_cgroup *root)
-{
-	/*
-	 * When a group in the hierarchy below root is destroyed, the
-	 * hierarchy iterator can no longer be trusted since it might
-	 * have pointed to the destroyed group.  Invalidate it.
-	 */
-	atomic_inc(&root->dead_count);
-}
-
-static struct mem_cgroup *
-mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
-		     struct mem_cgroup *root,
-		     int *sequence)
-{
-	struct mem_cgroup *position = NULL;
-	/*
-	 * A cgroup destruction happens in two stages: offlining and
-	 * release.  They are separated by a RCU grace period.
-	 *
-	 * If the iterator is valid, we may still race with an
-	 * offlining.  The RCU lock ensures the object won't be
-	 * released, tryget will fail if we lost the race.
-	 */
-	*sequence = atomic_read(&root->dead_count);
-	if (iter->last_dead_count == *sequence) {
-		smp_rmb();
-		position = iter->last_visited;
-
-		/*
-		 * We cannot take a reference to root because we might race
-		 * with root removal and returning NULL would end up in
-		 * an endless loop on the iterator user level when root
-		 * would be returned all the time.
-		 */
-		if (position && position != root &&
-		    !css_tryget_online(&position->css))
-			position = NULL;
-	}
-	return position;
-}
-
-static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
-				   struct mem_cgroup *last_visited,
-				   struct mem_cgroup *new_position,
-				   struct mem_cgroup *root,
-				   int sequence)
-{
-	/* root reference counting symmetric to mem_cgroup_iter_load */
-	if (last_visited && last_visited != root)
-		css_put(&last_visited->css);
-	/*
-	 * We store the sequence count from the time @last_visited was
-	 * loaded successfully instead of rereading it here so that we
-	 * don't lose destruction events in between.  We could have
-	 * raced with the destruction of @new_position after all.
-	 */
-	iter->last_visited = new_position;
-	smp_wmb();
-	iter->last_dead_count = sequence;
-}
-
 /**
  * mem_cgroup_iter - iterate over memory cgroup hierarchy
  * @root: hierarchy root
@@ -1204,8 +1081,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 				   struct mem_cgroup *prev,
 				   struct mem_cgroup_reclaim_cookie *reclaim)
 {
+	struct reclaim_iter *uninitialized_var(iter);
+	struct cgroup_subsys_state *css = NULL;
 	struct mem_cgroup *memcg = NULL;
-	struct mem_cgroup *last_visited = NULL;
+	struct mem_cgroup *pos = NULL;
 
 	if (mem_cgroup_disabled())
 		return NULL;
@@ -1214,50 +1093,101 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		root = root_mem_cgroup;
 
 	if (prev && !reclaim)
-		last_visited = prev;
+		pos = prev;
 
 	if (!root->use_hierarchy && root != root_mem_cgroup) {
 		if (prev)
-			goto out_css_put;
+			goto out;
 		return root;
 	}
 
 	rcu_read_lock();
-	while (!memcg) {
-		struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
-		int uninitialized_var(seq);
 
 	if (reclaim) {
 		struct mem_cgroup_per_zone *mz;
 
 		mz = mem_cgroup_zone_zoneinfo(root, reclaim->zone);
-			iter = &mz->reclaim_iter[reclaim->priority];
-			if (prev && reclaim->generation != iter->generation) {
-				iter->last_visited = NULL;
+		iter = &mz->iter[reclaim->priority];
+
+		if (prev && reclaim->generation != iter->generation)
 			goto out_unlock;
+
+		do {
+			pos = ACCESS_ONCE(iter->position);
+			/*
+			 * A racing update may change the position and
+			 * put the last reference, hence css_tryget(),
+			 * or retry to see the updated position.
+			 */
+		} while (pos && !css_tryget(&pos->css));
+	}
+
+	if (pos)
+		css = &pos->css;
+
+	for (;;) {
+		css = css_next_descendant_pre(css, &root->css);
+		if (!css) {
+			/*
+			 * Reclaimers share the hierarchy walk, and a
+			 * new one might jump in right at the end of
+			 * the hierarchy - make sure they see at least
+			 * one group and restart from the beginning.
+			 */
+			if (!prev)
+				continue;
+			break;
 		}
 
-			last_visited = mem_cgroup_iter_load(iter, root, &seq);
+		/*
+		 * Verify the css and acquire a reference.  The root
+		 * is provided by the caller, so we know it's alive
+		 * and kicking, and don't take an extra reference.
+		 */
+		memcg = mem_cgroup_from_css(css);
+
+		if (css == &root->css)
+			break;
+
+		if (css_tryget_online(css)) {
+			/*
+			 * Make sure the memcg is initialized:
+			 * mem_cgroup_css_online() orders the the
+			 * initialization against setting the flag.
+			 */
+			if (smp_load_acquire(&memcg->initialized))
+				break;
+
+			css_put(css);
 		}
 
-		memcg = __mem_cgroup_iter_next(root, last_visited);
+		memcg = NULL;
+	}
 
 	if (reclaim) {
-			mem_cgroup_iter_update(iter, last_visited, memcg, root,
-					seq);
+		if (cmpxchg(&iter->position, pos, memcg) == pos) {
+			if (memcg)
+				css_get(&memcg->css);
+			if (pos)
+				css_put(&pos->css);
+		}
+
+		/*
+		 * pairs with css_tryget when dereferencing iter->position
+		 * above.
+		 */
+		if (pos)
+			css_put(&pos->css);
 
 		if (!memcg)
 			iter->generation++;
-			else if (!prev && memcg)
+		else if (!prev)
 			reclaim->generation = iter->generation;
 	}
 
-		if (prev && !memcg)
-			goto out_unlock;
-	}
 out_unlock:
 	rcu_read_unlock();
-out_css_put:
+out:
 	if (prev && prev != root)
 		css_put(&prev->css);
 
@@ -5447,24 +5377,6 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	return 0;
 }
 
-/*
- * Announce all parents that a group from their hierarchy is gone.
- */
-static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
-{
-	struct mem_cgroup *parent = memcg;
-
-	while ((parent = parent_mem_cgroup(parent)))
-		mem_cgroup_iter_invalidate(parent);
-
-	/*
-	 * if the root memcg is not hierarchical we have to check it
-	 * explicitely.
-	 */
-	if (!root_mem_cgroup->use_hierarchy)
-		mem_cgroup_iter_invalidate(root_mem_cgroup);
-}
-
 static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
@@ -5485,8 +5397,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 
 	kmem_cgroup_css_offline(memcg);
 
-	mem_cgroup_invalidate_reclaim_iterators(memcg);
-
 	/*
 	 * This requires that offlining is serialized.  Right now that is
 	 * guaranteed because css_killed_work_fn() holds the cgroup_mutex.