linux-kernel - Re: [External] Re: [PATCH 4/5] mm: memcg/slab: Fix root memcg vmstats

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Message-ID: <20201029001408.GF827280@carbon.dhcp.thefacebook.com>
Date:   Wed, 28 Oct 2020 17:14:08 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, <laoar.shao@...il.com>,
        Chris Down <chris@...isdown.name>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>, <esyr@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>, <areber@...hat.com>,
        Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [External] Re: [PATCH 4/5] mm: memcg/slab: Fix root memcg vmstats

On Wed, Oct 28, 2020 at 10:56:20AM +0800, Muchun Song wrote:
> On Wed, Oct 28, 2020 at 2:48 AM Roman Gushchin <guro@...com> wrote:
> >
> > On Tue, Oct 27, 2020 at 04:02:55PM +0800, Muchun Song wrote:
> > > If we reparent the slab objects to the root memcg, when we free
> > > the slab object, we need to update the per-memcg vmstats to keep
> > > it correct for the root memcg. Now this at least affects the vmstat
> > > of NR_KERNEL_STACK_KB for !CONFIG_VMAP_STACK when the thread stack
> > > size is smaller than the PAGE_SIZE.
> > >
> > > Fixes: ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes")
> > > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> >
> > Can you, please, drop this patch for now?
> >
> > I'm working on a bigger cleanup related to the handling of the root memory
> > cgroup (I sent a link earlier in this thread), which already does a similar change.
> > There are several issues like this one, so it will be nice to fix them all at once.
> 
> I have read the patch of https://lkml.org/lkml/2020/10/14/869. You
> mean this patch
> fixes this issue? It chooses to uncharge the root memcg. But here we may need to
> uncharge the root memcg to keep root vmstats correct. If we do not do
> this, we can
> see the wrong vmstats via root memory.stat(e.g. NR_KERNEL_STACK_KB).

I pointed at a different patch in the same thread (it looks like you read the first one):
https://lkml.org/lkml/2020/10/21/612

It contained the following part:

@@ -868,7 +860,7 @@ void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
 	memcg = mem_cgroup_from_obj(p);
 
 	/* Untracked pages have no memcg, no lruvec. Update only the node */
-	if (!memcg || memcg == root_mem_cgroup) {
+	if (!memcg) {
 		__mod_node_page_state(pgdat, idx, val);
 	} else {
 		lruvec = mem_cgroup_lruvec(memcg, pgdat);

So it's exactly what your patch does.

Thanks!