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t=1794473654; bh=OT4UYzJ55zbPPzE3sSYyUxZKuwkGev+001urY+Jtyv0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HSxzzXAQAXajBmw3uzuM1xXEfDxqd9aJG7PLJu051MyVrGQ1NaJm3jkY2BfPcSs61 KQVrEw+f/uZxjoPuRgJso2EKttZxcRCo50io39UiuYuf4Il8I1iDf4lFu29Je5hkke wMxFFXLGjAE0evcUlapK2syy4lY2aGUBLtmwfUcQP5TEPNcCO9sC/3Oe+1isVNwt6l Oe6xf2kLohBkQTpw7HlPYlVGuPoUTVTpLqvCRH7aIGvPwbYKHiCVA5kXV0VH7M8nio zhe7wrGIvhQ6/RoYNJw700AstBFvZkICaw2fkm+pVWHqezWETqQZc1oun5vXPru4x1 ZsuSAGWiWPm2Q== Message-ID: Date: Thu, 12 Nov 2026 09:54:03 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: don't move memcg NR_FILE_PAGES for swapcache To: Song Hu Cc: akpm@linux-foundation.org, apopple@nvidia.com, byungchul@sk.com, gourry@gourry.net, joshua.hahnjy@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, matthew.brost@intel.com, ridong.chen@linux.dev, shivankg@amd.com, willy@infradead.org, ying.huang@linux.alibaba.com, ziy@nvidia.com References: <67275d3d-cc60-406c-a088-298b948d7577@kernel.org> <20261112024855.424402-1-husong@kylinos.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 1B85440004 X-Stat-Signature: hkq5jif485k9b4qttxde4rq1hzqq41r7 X-Rspam-User: X-HE-Tag: 1794473654-981612 X-HE-Meta: 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 964VSW9f LKAJIDmDPXATs/OkQy9TjQdtN5g+iU5Px+joPAoThYFy+CH3eB8mRovQ1+wANOaoJI+1DowQa0nmoiIAuvm4jDzT8N+YgCk6fosb4XcCPhlH446eF6W5msYdE5pc68l9/5BygbfunpxwP/ZjAuzl+RAmHeWb1dWWOeGO6Ru78yyi8FthL/QJI/6sCqSSOnkCSrlpP5lgxafAFhe5LdhUDF+dmn8xgCY4DIc251PgXoWA+L+vCjTzn2VqdfwtDHcdct8lLY6nblA+kttWta1XQo2PWZSx/Vsez5HX87sC9er7FDmcPubcrpSPkZAAwEjBwB2OHpsD7kpXMpYipP6egYELlp+HtMC1MYudj8SiTyGR03yljoUZOCU2vI5uT/05raSKGUIp2gwx4e2r1j6w5YwgkH+Ka5drmPNJHZ1tNZZt0XWw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/12/26 03:48, Song Hu wrote: >> On 11/10/26 04:09, Zi Yan wrote: >>> >>> It would be better to simplify the above like Joshua suggested. >>> >>> >>> Like Joshua said in another email, it will be more readable if >>> NR_SWAPCACHE updates can be moved into the then branch above. >> >> Can one of you write me the updated patch description? Thanks1 > > Sure. Here's an updated commit message incorporating Joshua's > suggestion to describe the accounting model first and then explain why > migration gets it wrong. > > mm/migrate: don't move memcg NR_FILE_PAGES for swapcache > > NR_FILE_PAGES is accounted on both the node and memcg planes for page > cache folios. Swapcache folios are the exception: they keep > NR_FILE_PAGES only on the node plane, while their memcg accounting is > tracked as NR_SWAPCACHE. > > The cross-zone path in __folio_migrate_mapping() currently migrates > NR_FILE_PAGES through mod_lruvec_state(), which updates both planes. > This is correct for normal page cache, but wrong for swapcache folios > because there is no memcg NR_FILE_PAGES state to move. As a result, > migration incorrectly increments the destination memcg counter and > decrements the source one, leaving the per-memcg accounting incorrect > until the memcg is destroyed. > > Reproducer, on a 2-node QEMU guest with cgroup v2: > > 1. create a cgroup with memory.max = 50M and swap enabled; > 2. memset 128M of anonymous memory in it, so reclaim swaps most of > it out; > 3. fork to raise the swap entry refcount, then fault the memory > back in, so the resident folios are swapcache ones; > 4. migrate_pages(pid, node0 -> node1). > > "file" row of memory.numa_stat (bytes) after step 4: > > without this patch with this patch > N0 N1 N0 N1 > 0 (*) 51691520 0 0 > > (*) the source counter has gone negative; negative values read as 0. > > Fix it by updating the node accounting directly for swapcache folios > while leaving the nonexistent memcg NR_FILE_PAGES state untouched. > Non-swapcache folios continue to use mod_lruvec_state(), preserving > the existing behaviour. This mirrors the existing swapcache handling > for NR_SHMEM in the same block. It accompanies > commit 4eef27af2932 ("mm/migrate: move NR_FILE_THPS on cross-node > migration") and commit c5eef2e03618 ("mm/migrate: move > NR_KERNEL_FILE_PAGES on cross-node migration"). > > Reviewed-by: Joshua Hahn > Acked-by: Zi Yan > Signed-off-by: Song Hu > > Thanks to Joshua and Zi for the helpful reviews. Updated, thanks! -- Cheers, David