Commit ec0db74b authored by Kinsey Ho's avatar Kinsey Ho Committed by Andrew Morton
Browse files

mm: restart if multiple traversals raced 

Currently, if multiple reclaimers raced on the same position, the
reclaimers which detect the race will still reclaim from the same memcg. 
Instead, the reclaimers which detect the race should move on to the next
memcg in the hierarchy.

So, in the case where multiple traversals race, jump back to the start of
the mem_cgroup_iter() function to find the next memcg in the hierarchy to
reclaim from.

Link: https://lkml.kernel.org/r/20240905003058.1859929-5-kinseyho@google.com


Reported-by: default avatar <syzbot+e099d407346c45275ce9@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/000000000000817cf10620e20d33@google.com/


Signed-off-by: default avatarKinsey Ho <kinseyho@google.com>
Reviewed-by: default avatarT.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 3d150e31

include/linux/memcontrol.h

+2 −2
Original line number Diff line number Diff line
@@ -57,7 +57,7 @@ enum memcg_memory_event { 


struct mem_cgroup_reclaim_cookie {

	pg_data_t *pgdat;

	unsigned int generation;

	int generation;

};



#ifdef CONFIG_MEMCG
@@ -78,7 +78,7 @@ struct lruvec_stats; 
struct mem_cgroup_reclaim_iter {

	struct mem_cgroup *position;

	/* scan generation, increased every round-trip */

	unsigned int generation;

	atomic_t generation;

};



/*

mm/memcontrol.c

+17 −9
Original line number Diff line number Diff line
@@ -986,8 +986,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, 
				   struct mem_cgroup_reclaim_cookie *reclaim)

{

	struct mem_cgroup_reclaim_iter *iter;

	struct cgroup_subsys_state *css = NULL;

	struct mem_cgroup *memcg = NULL;

	struct cgroup_subsys_state *css;

	struct mem_cgroup *memcg;

	struct mem_cgroup *pos = NULL;



	if (mem_cgroup_disabled())
@@ -998,19 +998,23 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, 


	rcu_read_lock();

restart:

	memcg = NULL;



	if (reclaim) {

		int gen;

		struct mem_cgroup_per_node *mz;



		mz = root->nodeinfo[reclaim->pgdat->node_id];

		iter = &mz->iter;

		gen = atomic_read(&iter->generation);



		/*

		 * On start, join the current reclaim iteration cycle.

		 * Exit when a concurrent walker completes it.

		 */

		if (!prev)

			reclaim->generation = iter->generation;

		else if (reclaim->generation != iter->generation)

			reclaim->generation = gen;

		else if (reclaim->generation != gen)

			goto out_unlock;



		pos = READ_ONCE(iter->position);
@@ -1018,8 +1022,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, 
		pos = prev;

	}



	if (pos)

		css = &pos->css;

	css = pos ? &pos->css : NULL;



	for (;;) {

		css = css_next_descendant_pre(css, &root->css);
@@ -1033,21 +1036,26 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, 
		 * and kicking, and don't take an extra reference.

		 */

		if (css == &root->css || css_tryget(css)) {

			memcg = mem_cgroup_from_css(css);

			break;

		}

	}



	memcg = mem_cgroup_from_css(css);



	if (reclaim) {

		/*

		 * The position could have already been updated by a competing

		 * thread, so check that the value hasn't changed since we read

		 * it to avoid reclaiming from the same cgroup twice.

		 */

		(void)cmpxchg(&iter->position, pos, memcg);

		if (cmpxchg(&iter->position, pos, memcg) != pos) {

			if (css && css != &root->css)

				css_put(css);

			goto restart;

		}



		if (!memcg) {

			iter->generation++;

			atomic_inc(&iter->generation);



			/*

			 * Reclaimers share the hierarchy walk, and a