Re: [RFC PATCH] mm, memcg: fix (Re: OOM: Better, but still there on)

[Michal Hocko]

On Fri 30-12-16 10:19:26, Mel Gorman wrote:
> On Mon, Dec 26, 2016 at 01:48:40PM +0100, Michal Hocko wrote:
> > On Fri 23-12-16 23:26:00, Nils Holland wrote:
> > > On Fri, Dec 23, 2016 at 03:47:39PM +0100, Michal Hocko wrote:
> > > > 
> > > > Nils, even though this is still highly experimental, could you give it a
> > > > try please?
> > > 
> > > Yes, no problem! So I kept the very first patch you sent but had to
> > > revert the latest version of the debugging patch (the one in
> > > which you added the "mm_vmscan_inactive_list_is_low" event) because
> > > otherwise the patch you just sent wouldn't apply. Then I rebooted with
> > > memory cgroups enabled again, and the first thing that strikes the eye
> > > is that I get this during boot:
> > > 
> > > [    1.568174] ------------[ cut here ]------------
> > > [    1.568327] WARNING: CPU: 0 PID: 1 at mm/memcontrol.c:1032 mem_cgroup_update_lru_size+0x118/0x130
> > > [    1.568543] mem_cgroup_update_lru_size(f4406400, 2, 1): lru_size 0 but not empty
> > 
> > Ohh, I can see what is wrong! a) there is a bug in the accounting in
> > my patch (I double account) and b) the detection for the empty list
> > cannot work after my change because per node zone will not match per
> > zone statistics. The updated patch is below. So I hope my brain already
> > works after it's been mostly off last few days...
> > ---
> > From 397adf46917b2d9493180354a7b0182aee280a8b Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > Date: Fri, 23 Dec 2016 15:11:54 +0100
> > Subject: [PATCH] mm, memcg: fix the active list aging for lowmem requests when
> >  memcg is enabled
> > 
> > Nils Holland has reported unexpected OOM killer invocations with 32b
> > kernel starting with 4.8 kernels
> > 
> 
> I think it's unfortunate that per-zone stats are reintroduced to the
> memcg structure.

the original patch I had didn't add per zone stats but rather did a
nr_highmem counter to mem_cgroup_per_node (inside ifdeff CONFIG_HIGMEM).
This would help for this particular case but it wouldn't work for other
lowmem requests (e.g. GFP_DMA32) and with the kmem accounting this might
be a problem in future. So I've decided to go with a more generic
approach which requires per-zone tracking. I cannot say I would be
overly happy about this at all.

> I can't help but think that it would have also worked
> to always rotate a small number of pages if !inactive_list_is_low and
> reclaiming for memcg even if it distorted page aging.

I am not really sure how that would work. Do you mean something like the
following?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fa30010a5277..563ada3c02ac 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2044,6 +2044,9 @@ static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
 	inactive = lruvec_lru_size(lruvec, file * LRU_FILE);
 	active = lruvec_lru_size(lruvec, file * LRU_FILE + LRU_ACTIVE);
 
+	if (!mem_cgroup_disabled())
+		goto out;
+
 	/*
 	 * For zone-constrained allocations, it is necessary to check if
 	 * deactivations are required for lowmem to be reclaimed. This
@@ -2063,6 +2066,7 @@ static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
 		active -= min(active, active_zone);
 	}
 
+out:
 	gb = (inactive + active) >> (30 - PAGE_SHIFT);
 	if (gb)
 		inactive_ratio = int_sqrt(10 * gb);

The problem I see with such an approach is that chances are that this
would reintroduce what f8d1a31163fc ("mm: consider whether to decivate
based on eligible zones inactive ratio") tried to fix. But maybe I have
missed your point.

> However, given that such an approach would be less robust and this has
> been heavily tested;
> 
> Acked-by: Mel Gorman <mgorman@xxxxxxx>

Thanks!
-- 
Michal Hocko
SUSE Labs