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Date: Thu, 29 Jan 2026 10:50:15 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Shakeel Butt <shakeel.butt@linux.dev>
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: <20260129105015.0ed043764a4ad220334b7ae1@linux-foundation.org>
In-Reply-To: <20260129184054.910897-1-shakeel.butt@linux.dev>
References: <20260129184054.910897-1-shakeel.butt@linux.dev>
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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?

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

Should we cc:stable?