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Wed, 28 Oct 2026 13:30:01 +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 R8HKH9n44WrwOgAAD6G6ig (envelope-from ); Wed, 28 Oct 2026 13:30:01 +0000 Date: Wed, 28 Oct 2026 13:30:00 +0000 From: Pedro Falcato To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, Chris Mason , stable@vger.kernel.org Subject: Re: [PATCH 17/46] mm/khugepaged: preserve dirty bits when collapsing PTE-mapped THPs Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-17-6f74bec9a5bb@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: <20261027-fixes_core-v1-17-6f74bec9a5bb@kernel.org> X-Spamd-Result: default: False [-11.80 / 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]; MIME_GOOD(-0.10)[text/plain]; MISSING_XM_UA(0.00)[]; TAGGED_RCPT(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[linux-foundation.org,cmpxchg.org,kernel.org,linux.dev,tencent.com,google.com,linux.alibaba.com,infradead.org,suse.com,soleen.com,suse.cz,nvidia.com,surriel.com,arm.com,intel.com,redhat.com,gmail.com,suse.de,huaweicloud.com,lge.com,gourry.net,sk.com,ziepe.ca,kvack.org,vger.kernel.org,lists.infradead.org]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_SOME(0.00)[]; RCPT_COUNT_GT_50(0.00)[65]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:email] X-Spam-Score: -11.80 X-Spam-Level: X-Spam-Flag: NO On Tue, Oct 27, 2026 at 12:17:37AM +0100, David Hildenbrand (Arm) wrote: > From: Chris Mason > > try_collapse_pte_mapped_thp() clears PTEs without transferring their dirty > bits to the folio. A shmem read fault can map a clean folio writable, so > a later store only dirties the PTE. Losing that dirty bit allows reclaim > to discard modified data. > > The assumption that regular files are read-only is also obsolete since > commit 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse > for writable files"). In any case, transferring PTE dirty bits to the folio > when removing mappings (and while the folio is still mapped) is the right > thing to do. > > Use get_and_clear_ptes() to collect the dirty bits and mark the folio > dirty before unmapping it. Update the comment. > > This was reproduced with shmem using MADV_COLLAPSE followed by > MADV_PAGEOUT. > > Fix generated by an LLM and sent by Chris Mason in private. > > Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP") > Fixes: 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse for writable files") > Cc: stable@vger.kernel.org > Signed-off-by: Chris Mason > [ david: adjust comment; simplify description; cover file-thp as well ] > Signed-off-by: David Hildenbrand (Arm) Reviewed-by: Pedro Falcato -- Pedro