linux-kernel - Re: [PATCH v2] mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in collapse

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Message-ID: <aXzmgp84VD0Vt3KC@thinkstation>
Date: Fri, 30 Jan 2026 17:15:20 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Rik van Riel <riel@...riel.com>, 
	Song Liu <songliubraving@...com>, 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>, Matthew Wilcox <willy@...radead.org>, 
	Meta kernel team <kernel-team@...a.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in
 collapse_file()

On Thu, Jan 29, 2026 at 08:29:25PM -0800, Shakeel Butt 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")

My bug survived for almost 10 years!

> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Reviewed-by: Kiryl Shutsemau <kas@...nel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov