linux-kernel - [PATCH 2/3] mm/memcg: fix endless iteration in reclaim

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 13 Jan 2014 17:52:30 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim

On one home machine I can easily reproduce (by rmdir of memcgdir during
reclaim) multiple processes stuck looping forever in mem_cgroup_iter():
__mem_cgroup_iter_next() keeps selecting the memcg being destroyed, fails
to tryget it, returns NULL to mem_cgroup_iter(), which goes around again.

It's better to err on the side of leaving the loop too soon than never
when such races occur: once we've served prev (using root if none),
get out the next time __mem_cgroup_iter_next() cannot deliver.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
Securing the tree iterator against such races is difficult, I've
certainly got it wrong myself before.  Although the bug is real, and
deserves a Cc stable, you may want to play around with other solutions
before committing to this one.  The current iterator goes back to v3.12:
I'm really not sure if v3.11 was good or not - I never saw the problem
in the vanilla kernel, but with Google mods in we also had to make an
adjustment, there to stop __mem_cgroup_iter() being called endlessly
from the reclaim level.

 mm/memcontrol.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- mmotm/mm/memcontrol.c	2014-01-10 18:25:02.236448954 -0800
+++ linux/mm/memcontrol.c	2014-01-12 22:21:10.700570471 -0800
@@ -1254,8 +1252,11 @@ struct mem_cgroup *mem_cgroup_iter(struc
 				reclaim->generation = iter->generation;
 		}
 
-		if (prev && !memcg)
+		if (!memcg) {
+			if (!prev)
+				memcg = root;
 			goto out_unlock;
+		}
 	}
 out_unlock:
 	rcu_read_unlock();
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/