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Date: Sat, 31 Jan 2026 13:15:11 -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>, Matthew Wilcox
 <willy@infradead.org>, Meta kernel team <kernel-team@meta.com>,
 linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in
 collapse_file()
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References: <20260130042925.2797946-1-shakeel.butt@linux.dev>
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On Thu, 29 Jan 2026 20:29:25 -0800 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> In META's fleet, we observed high-level cgroups showing zero file memcg
> stats while their descendants had non-zero values. Investigation using
> drgn revealed that these parent cgroups actually had negative file stats,
> aggregated from their children.
> 
> This issue became more frequent after deploying thp-always more widely,
> pointing to a correlation with THP file collapsing. The root cause is
> that collapse_file() assumes old folios and the new THP belong to the
> same node and memcg. When this assumption breaks, stats become skewed.
> The bug affects not just memcg stats but also per-numa stats, and not
> just NR_FILE_PAGES but also NR_SHMEM.
> 
> The assumption breaks in scenarios such as:
> 
> 1. Small folios allocated on one node while the THP gets allocated on a
>    different node.
> 
> 2. A package downloader running in one cgroup populates the page cache,
>    while a job in a different cgroup executes the downloaded binary.
> 
> 3. A file shared between processes in different cgroups, where one
>    process faults in the pages and khugepaged (or madvise(COLLAPSE))
>    collapses them on behalf of the other.
> 
> Fix the accounting by explicitly incrementing stats for the new THP and
> decrementing stats for the old folios being replaced.
> 
> Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")

As the bug is 10 years old I think I'll queue this for 6.20(?)-rc1 with
cc:stable.  Just to get it a bit more time-under-test before -stable
kernels pick it up.  Sound OK?