linux-kernel - Re: [PATCH] mm: khugepaged: fix NR_FILE_PAGES accounting in collapse

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Message-ID: <aXusxKTW5Y6-0qKO@linux.dev>
Date: Thu, 29 Jan 2026 10:55:14 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Rik van Riel <riel@...riel.com>, 
	Song Liu <songliubraving@...com>, Kiryl Shutsemau <kas@...nel.org>, 
	Usama Arif <usamaarif642@...il.com>, David Hildenbrand <david@...nel.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>, 
	Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>, 
	Meta kernel team <kernel-team@...a.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix NR_FILE_PAGES accounting in
 collapse_file()

On Thu, Jan 29, 2026 at 10:50:15AM -0800, Andrew Morton wrote:
> On Thu, 29 Jan 2026 10:40:54 -0800 Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> 
> > In META's fleet, we are seeing high level cgroups with zero file memcg
> > stat but their descendants have non-zero file stat. This should not be
> > possible. On further inspection by looking at kernel data structures
> > though drgn, it was revealed that the high level cgroups have negative
> > file stat which was aggregated from their children.
> > 
> > Another interesting point was that this specific issue start happening
> > more often as we started deploying thp-always more widely which
> > indicates some correlation between file memory and THPs and indeed it
> > was found that file memcg stat accounting is buggy in the collapse code
> > path from the start.
> 
> So this has no known runtime effect apart from incorrect accounting?

Yes just an accounting bug.

> 
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> 
> Should we cc:stable?

I think so.