linux-kernel - Re: [PATCH for 3.2.34] memcg: do not trigger OOM from add_to_page_cache

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Message-ID: <20121217163203.GD25432@dhcp22.suse.cz>
Date:	Mon, 17 Dec 2012 17:32:03 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	azurIt <azurit@...ox.sk>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups mailinglist <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH for 3.2.34] memcg: do not trigger OOM from
 add_to_page_cache_locked

On Mon 17-12-12 02:34:30, azurIt wrote:
> >I would try to limit changes to minimum. So the original kernel you were
> >using + the first patch to prevent OOM from the write path + 2 debugging
> >patches.
> 
> 
> It didn't take off the whole system this time (but i was
> prepared to record a video of console ;) ), here it is:
> http://www.watchdog.sk/lkml/oom_mysqld4

[...]
[ 1248.059429] ------------[ cut here ]------------
[ 1248.059586] WARNING: at mm/memcontrol.c:2400 T.1146+0x2d9/0x610()
[ 1248.059723] Hardware name: S5000VSA
[ 1248.059855] gfp_mask:208 nr_pages:1 oom:0 ret:2

This is GFP_KERNEL allocation which is expected. It is also a simple
page which is not that expected because we shouldn't return ENOMEM on
those unless this was GFP_ATOMIC allocation (which it wasn't) or the
caller told us to not trigger OOM which is the case only for THP pages
(see mem_cgroup_charge_common). So the big question is how have we ended
up with oom=false here...

[Ohh, I am really an idiot. I screwed the first patch]
-       bool oom = true;
+       bool oom = !(gfp_mask | GFP_MEMCG_NO_OOM);

Which obviously doesn't work. It should read !(gfp_mask &GFP_MEMCG_NO_OOM).
  No idea how I could have missed that. I am really sorry about that.
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c04676d..1f35a74 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2704,7 +2704,7 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 	struct mem_cgroup *memcg = NULL;
 	unsigned int nr_pages = 1;
 	struct page_cgroup *pc;
-	bool oom = !(gfp_mask | GFP_MEMCG_NO_OOM);
+	bool oom = !(gfp_mask & GFP_MEMCG_NO_OOM);
 	int ret;
 
 	if (PageTransHuge(page)) {
-- 
Michal Hocko
SUSE Labs
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