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Date: Thu, 29 Jan 2026 10:55:14 -0800
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From: Shakeel Butt <shakeel.butt@linux.dev>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Rik van Riel <riel@surriel.com>, 
	Song Liu <songliubraving@fb.com>, Kiryl Shutsemau <kas@kernel.org>, 
	Usama Arif <usamaarif642@gmail.com>, David Hildenbrand <david@kernel.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@oracle.com>, Zi Yan <ziy@nvidia.com>, 
	Baolin Wang <baolin.wang@linux.alibaba.com>, "Liam R . Howlett" <Liam.Howlett@oracle.com>, 
	Nico Pache <npache@redhat.com>, Ryan Roberts <ryan.roberts@arm.com>, Dev Jain <dev.jain@arm.com>, 
	Barry Song <baohua@kernel.org>, Lance Yang <lance.yang@linux.dev>, 
	Meta kernel team <kernel-team@meta.com>, linux-mm@kvack.org, cgroups@vger.kernel.org, 
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix NR_FILE_PAGES accounting in
 collapse_file()
Message-ID: <aXusxKTW5Y6-0qKO@linux.dev>
References: <20260129184054.910897-1-shakeel.butt@linux.dev>
 <20260129105015.0ed043764a4ad220334b7ae1@linux-foundation.org>
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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@linux.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.