Re: [PATCH] mm: memcontrol: fix possible memcg leak due to interrupted reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Dec 12, 2015 at 04:34:02PM +0300, Vladimir Davydov wrote:
> Memory cgroup reclaim can be interrupted with mem_cgroup_iter_break()
> once enough pages have been reclaimed, in which case, in contrast to a
> full round-trip over a cgroup sub-tree, the current position stored in
> mem_cgroup_reclaim_iter of the target cgroup does not get invalidated
> and so is left holding the reference to the last scanned cgroup. If the
> target cgroup does not get scanned again (we might have just reclaimed
> the last page or all processes might exit and free their memory
> voluntary), we will leak it, because there is nobody to put the
> reference held by the iterator.
> 
> The problem is easy to reproduce by running the following command
> sequence in a loop:
> 
>     mkdir /sys/fs/cgroup/memory/test
>     echo 100M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
>     echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
>     memhog 150M
>     echo $$ > /sys/fs/cgroup/memory/cgroup.procs
>     rmdir test
> 
> The cgroups generated by it will never get freed.
> 
> This patch fixes this issue by making mem_cgroup_iter_break() clear
> mem_cgroup_reclaim_iter->position in case it points to the memory cgroup
> we interrupted reclaim on.
> 
> Fixes: 5ac8fb31ad2e ("mm: memcontrol: convert reclaim iterator to simple css refcounting")
> Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: <stable@vger.kernel.org> # 3.19+

Good catch!

The downside of not remembering the last position across incomplete
reclaim cycles is that we always restart at the same position. If a
cgroup has a certain number of children, it's conceivable that we
might never get to the youngest cgroups in the subtree anymore.

So I'd be more comfortable removing incomplete reclaim walks. It was a
nice little optimization we could do while supporting interleave walks,
but it's not necessary: when global reclaim can walk the entire system,
limit reclaim should be okay walking subtrees exhaustively.

How about this?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 34f4a14..62a4731 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -933,6 +933,9 @@ out:
  * mem_cgroup_iter_break - abort a hierarchy walk prematurely
  * @root: hierarchy root
  * @prev: last visited hierarchy member as returned by mem_cgroup_iter()
+ *
+ * Note: do not use this from a shared iterator walk (when using a
+ * reclaim cookie), the iterator state may leak the css reference!
  */
 void mem_cgroup_iter_break(struct mem_cgroup *root,
 			   struct mem_cgroup *prev)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2dbc679..41b07b2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2425,21 +2425,6 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
 				   sc->nr_scanned - scanned,
 				   sc->nr_reclaimed - reclaimed);
 
-			/*
-			 * Direct reclaim and kswapd have to scan all memory
-			 * cgroups to fulfill the overall scan target for the
-			 * zone.
-			 *
-			 * Limit reclaim, on the other hand, only cares about
-			 * nr_to_reclaim pages to be reclaimed and it will
-			 * retry with decreasing priority if one round over the
-			 * whole hierarchy is not sufficient.
-			 */
-			if (!global_reclaim(sc) &&
-					sc->nr_reclaimed >= sc->nr_to_reclaim) {
-				mem_cgroup_iter_break(root, memcg);
-				break;
-			}
 		} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
 
 		/*

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