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

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Date:   Tue, 27 Oct 2020 11:48:35 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <vdavydov.dev@...il.com>, <akpm@...ux-foundation.org>,
        <shakeelb@...gle.com>, <iamjoonsoo.kim@....com>,
        <laoar.shao@...il.com>, <chris@...isdown.name>,
        <christian.brauner@...ntu.com>, <peterz@...radead.org>,
        <mingo@...nel.org>, <keescook@...omium.org>, <tglx@...utronix.de>,
        <esyr@...hat.com>, <surenb@...gle.com>, <areber@...hat.com>,
        <elver@...gle.com>, <linux-kernel@...r.kernel.org>,
        <cgroups@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 4/5] mm: memcg/slab: Fix root memcg vmstats

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.

Thank you!

> ---
>  mm/memcontrol.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 22b4fb941b54..70345b15b150 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -875,8 +875,13 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
>  	rcu_read_lock();
>  	memcg = mem_cgroup_from_obj(p);
>  
> -	/* Untracked pages have no memcg, no lruvec. Update only the node */
> -	if (!memcg || memcg == root_mem_cgroup) {
> +	/*
> +	 * Untracked pages have no memcg, no lruvec. Update only the
> +	 * node. 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.
> +	 */
> +	if (!memcg) {
>  		__mod_node_page_state(pgdat, idx, val);
>  	} else {
>  		lruvec = mem_cgroup_lruvec(memcg, pgdat);
> -- 
> 2.20.1
>