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Thu, 12 Nov 2026 11:27:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id N8F0H5ai9WraAwAAD6G6ig (envelope-from ); Thu, 12 Nov 2026 11:27:18 +0000 Date: Thu, 12 Nov 2026 11:27:16 +0000 From: Pedro Falcato 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 , "Matthew Wilcox (Oracle)" , Jan Kara , Nhat Pham , Johannes Weiner , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Usama Arif , Kairui Song , Balbir Singh , =?utf-8?B?SsOpcsO0bWU=?= 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 , stable@vger.kernel.org Subject: Re: [PATCH 02/13] mm: cachestat: count each page only once Message-ID: References: <20261111-mason-migrate-et-al-fixes-v1-0-3d2454362e8d@kernel.org> <20261111-mason-migrate-et-al-fixes-v1-2-3d2454362e8d@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261111-mason-migrate-et-al-fixes-v1-2-3d2454362e8d@kernel.org> X-Spamd-Result: default: False [-12.01 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCPT_COUNT_TWELVE(0.00)[43]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[kernel.org,infradead.org,google.com,suse.com,linux.dev,linux-foundation.org,suse.cz,gmail.com,cmpxchg.org,nvidia.com,intel.com,sk.com,gourry.net,linux.alibaba.com,tencent.com,redhat.com,linux.intel.com,gentwo.org,nxp.com,fb.com,kvack.org,vger.kernel.org,alien8.de]; TAGGED_RCPT(0.00)[]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; R_RATELIMIT(0.00)[to_ip_from(RLziudqfrzo6b7hzgpxksh1d9i)]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -12.01 X-Spam-Level: X-Rspamd-Queue-Id: 44CA61F792 On Wed, Nov 11, 2026 at 06:13:16PM +0000, Lorenzo Stoakes (ARM) wrote: > From: Chris Mason > > It turns out that filemap_cachestat() can unwittingly return incorrect > page counts, if large folios are present. > > This can occur due to underflow - nr_cache is nearly 2^64 on a 64-bit > kernel if the input range starts more than 64 pages into a folio that is > split during the call. > > It can also occur due to pages after a large folio being skipped if the > range starts in its second or later XArray slot and the call reschedules at > that folio. > > Finally it can be caused by pages being counted twice due to folios in the > range being split or collapsed during the call. > > filemap_cachestat() walks the page cache under the RCU read lock only, > and for each entry it counts the pages that lie in the range. It does > not track how far it has counted, so the counts are only right if the > walk returns each index once. > > But a large folio can be split during the walk, or small folios can be > collapsed into a large one, and the next entries then cover indices that > were already counted. > > Resolve the issue by tracking the current index as one past the last index > in the folio, and explicitly using xas_set() to set the current position in > the XArray. > > Additionally, add a check for folio_last_index reaching or exceeding > last_index and break out of the loop if so. > > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") > Cc: > Signed-off-by: Chris Mason > [ ljs: updated to use curr_index, fixed comments, updated commit message ] > Signed-off-by: Lorenzo Stoakes (ARM) Reviewed-by: Pedro Falcato -- Pedro