linux-kernel - Re: [PATCH v3 28/30 fix 1/2] mm: memcontrol: fix lruvec_stats->state

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Message-ID: <aXCIsLQMSdQ2GNc4@linux.dev>
Date: Wed, 21 Jan 2026 00:20:10 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: Muchun Song <muchun.song@...ux.dev>, hannes@...xchg.org, 
	hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev, david@...nel.org, 
	lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com, yosry.ahmed@...ux.dev, 
	imran.f.khan@...cle.com, kamalesh.babulal@...cle.com, axelrasmussen@...gle.com, 
	yuanchu@...gle.com, weixugc@...gle.com, chenridong@...weicloud.com, mkoutny@...e.com, 
	akpm@...ux-foundation.org, hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com, 
	lance.yang@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	cgroups@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v3 28/30 fix 1/2] mm: memcontrol: fix
 lruvec_stats->state_local reparenting

On Wed, Jan 21, 2026 at 11:43:50AM +0800, Qi Zheng wrote:
> 
> 
> On 1/21/26 2:47 AM, Shakeel Butt wrote:
> > On Tue, Jan 20, 2026 at 03:19:00PM +0800, Muchun Song wrote:
> > > 
> > > 
> > > > > No reparenting local stats for v2.
> > > > 
> > > > It seems that lruvec_stats->state_local (non-hierarchical) needs to be
> > > > relocated in both v1 and v2.
> > > 
> > > Here we might need to elaborate a bit. Specifically, in the function
> > > `count_shadow_nodes`, the use of `lruvec_page_state_local` to obtain
> > > LRU and SLAB pages seems to also require these logics to work correctly.
> > > For SLAB, it appears that the statistics here have already been
> > > problematic for a while since SLAB pages have been reparented, right?
> > > 
> > 
> > Thanks a lot, now it is clear and yes it seems like SLAB is problematic
> > but now I am wondering if it is really worth fixing. For LRU pages, how
> > about using lruvec_lru_size() defined in vmscan.c. That would at least
> > keep count_shadow_nodes() working irrespective of LRU reparenting.
> 
> Do you mean calling lruvec_lru_size() in count_shadow_nodes()?

Yes but I am mainly brainstorming. We can keep the reparenting local
stats for both v1 and v2 for now as it is not a performance critical
path. I am more worried about the stats update path where upward
traversal of memcg for CSS_DYING can be costly and I don't want that in
v2.

> But
> numa_stat interface also reads lruvec_stats->state and make it visible
> to the user.
> 

Not sure how this is relevant.