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Thu, 12 Nov 2026 08:15:42 -0800 (PST) Date: Thu, 12 Nov 2026 11:15:41 -0500 From: Johannes Weiner To: "Lorenzo Stoakes (ARM)" Cc: David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , David Rientjes , Shakeel Butt , Andrew Morton , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Nhat Pham , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Usama Arif , Kairui Song , Balbir Singh , =?iso-8859-1?B?Suly9G1l?= Glisse , Kiryl Shutsemau , Yu-cheng Yu , Dave Hansen , Dennis Zhou , Tejun Heo , Christoph Lameter , Peng Fan , Josef Bacik , Chris Mason , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Borislav Petkov (AMD)" , Mark Brown , "Dennis Zhou (Facebook)" , Rick Edgecombe Subject: Re: [PATCH 03/13] mm: cachestat: restart the walk after each entry Message-ID: References: <20261111-mason-migrate-et-al-fixes-v1-0-3d2454362e8d@kernel.org> <20261111-mason-migrate-et-al-fixes-v1-3-3d2454362e8d@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261111-mason-migrate-et-al-fixes-v1-3-3d2454362e8d@kernel.org> X-Stat-Signature: gne3r6w9apfpx8ut7gn3g9fxgyj767t8 X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 11A69C000A X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1794500144-181382 X-HE-Meta: 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 ZffTqo6P 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 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 Wed, Nov 11, 2026 at 06:13:17PM +0000, Lorenzo Stoakes (ARM) wrote: > From: Chris Mason > > cachestat() can leave cached pages out of its counts. This happens if > the entry that the call has just counted is replaced by a larger folio > that starts in the same XArray slot. > > The issue is that filemap_cachestat() walks the page cache under the RCU > read lock, so racing concurrent collapse/split is possible. > > Thus, an entry can be replaced immediately after the walk has returned > it. When that happens, the next xas_find() skips the sibling entries that > point to the slot of that entry, since ordinarily they belong to that > entry. > > However a concurrent collapse_file() or __filemap_add_folio() (when > replacing a shadow entry) can store a larger folio in the slot, in which > case the sibling entries belong to the new folio and the walk still skips > them. > > Consider: > > cachestat collapse > ========= ======== > count a 64-page folio in slot 8 > store a 512-page folio in slot 8, > with sibling entries in slots 9-15 > xas_find() starts at slot 8 > and skips slots 9-15 > /* 448 pages are never counted */ > > Also consider: > > filemap_cachestat() __filemap_add_folio() > =================== ===================== > counts the shadow entry in slot 0 > slot 0: a folio of 32 pages > slots 1-31: siblings of slot 0 > xas_find() skips slots 1-31 > /* 31 pages are never counted */ > > Essentially the fact that large folios might be present means we cannot > safely continue the XArray walk, and must reset the iterator each time. > > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") > Signed-off-by: Chris Mason > [ ljs: squashed 2 commits, fixed up comment in code, and commit message ] > Signed-off-by: Lorenzo Stoakes (ARM) Reviewed-by: Johannes Weiner