Re: [patch 0/7] improve memcg oom killer robustness v2

[Johannes Weiner]

Hi azur,

On Wed, Oct 09, 2013 at 08:44:50PM +0200, azurIt wrote:
> Joahnnes,
> 
> i'm very sorry to say it but today something strange happened.. :) i was just right at the computer so i noticed it almost immediately but i don't have much info. Server stoped to respond from the net but i was already logged on ssh which was working quite fine (only a little slow). I was able to run commands on shell but i didn't do much because i was afraid that it will goes down for good soon. I noticed few things:
>  - htop was strange because all CPUs were doing nothing (totally nothing)
>  - there were enough of free memory
>  - server load was about 90 and was raising slowly
>  - i didn't see ANY process in 'run' state
>  - i also didn't see any process with strange behavior (taking much CPU, memory or so) so it wasn't obvious what to do to fix it
>  - i started to kill Apache processes, everytime i killed some, CPUs did some work, but it wasn't fixing the problem
>  - finally i did 'skill -kill apache2' in shell and everything started to work
>  - server monitoring wasn't sending any data so i have no graphs
>  - nothing interesting in logs
> 
> I will send more info when i get some.

Somebody else reported a problem on the upstream patches as well.  Any
chance you can confirm the stacks of the active but not running tasks?

It sounds like they are stuck on a waitqueue, the question is which
one.  I forgot to disable OOM for __GFP_NOFAIL allocations, so they
could succeed and leak an OOM context.  task structs are not
reinitialized between alloc & free so a different task could later try
to oom trylock a memcg that has been freed, fail, and wait
indefinitely on the OOM waitqueue.  There might be a simpler
explanation but I can't think of anything right now.

But the OOM context is definitely being leaked, so please apply the
following for your next reboot:

---
 mm/memcontrol.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5aee2fa..83ad39b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2341,6 +2341,9 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	 */
 	if (!*ptr && !mm)
 		goto bypass;
+
+	if (gfp_mask & __GFP_NOFAIL)
+		oom = false;
 again:
 	if (*ptr) { /* css should be a valid one */
 		memcg = *ptr;
-- 
1.8.4

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