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Fri, 30 Jan 2026 08:13:05 -0800 (PST) Date: Fri, 30 Jan 2026 11:13:05 -0500 From: Johannes Weiner To: Shakeel Butt Cc: Andrew Morton , Rik van Riel , Song Liu , Kiryl Shutsemau , Usama Arif , David Hildenbrand , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Matthew Wilcox , Meta kernel team , 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() Message-ID: References: <20260130042925.2797946-1-shakeel.butt@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260130042925.2797946-1-shakeel.butt@linux.dev> X-Rspamd-Queue-Id: CAD634000A X-Stat-Signature: 88x86noph5fcsm4dkmpjtuair4d6ur19 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1769789587-385385 X-HE-Meta: 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 U0of1+jX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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") > Signed-off-by: Shakeel Butt Acked-by: Johannes Weiner