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bh=ex0bR+wqY48y7Ys0l1lS/C4xyqfg/lqStx93s8K9wXI=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793599557; v=1; x=1794204357; b=mnLeKo3JIlrjZ0v8csYIXQvs0MqjNYBAaha6FI+GsXgzsTBscKhu47xfdxei1GstkYX55TtU s7+ijDrBsSQikbWr4RhQIABec//XfnZQfXcLKbZqgwK2XEUTDJ36HqPkKEIRh6O+ifRMQgl5bW3 ABiUUGhU4PsBaXEJa27dL80Y= X-Envelope-To: linux-cxl@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 85932c5ab249e449; Mon, 02 Nov 2026 06:05:56 +0000 X-Mizu-Trace-ID: 85932c5ab249e449 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, ljs@kernel.org, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, baoquan.he@linux.dev, baolin.wang@linux.alibaba.com, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kees@kernel.org, peterz@infradead.org, jannh@google.com, pfalcato@suse.de, hughd@google.com, pasha.tatashin@soleen.com, pratyush@kernel.org, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, lance.yang@linux.dev, riel@surriel.com, harry@kernel.org, apopple@nvidia.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, usama.arif@linux.dev, kas@kernel.org, matthew.brost@intel.com, jhubbard@nvidia.com, peterx@redhat.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, osalvador@suse.de, minchan@kernel.org, muchun.song@linux.dev, xu.xin@linux.dev, chengming.zhou@linux.dev, chrisl@kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, youngjun.park@lge.com, gourry@gourry.net, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, jgg@ziepe.ca, 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, mason@kernel.org, stable@vger.kernel.org, usama.anjum@arm.com Subject: Re: [PATCH v3 17/47] mm/khugepaged: preserve dirty bits when collapsing PTE-mapped THPs Date: Mon, 2 Nov 2026 14:05:47 +0800 Message-ID: <20261102060547.39942-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261031-fixes_core-v3-17-d6437171ecc3@kernel.org> References: <20261031-fixes_core-v3-17-d6437171ecc3@kernel.org> X-Mailing-List: linux-cxl@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=UTF-8 Content-Transfer-Encoding: 8bit On Sat, Oct 31, 2026 at 10:39:46AM +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 ] >Reviewed-by: Pedro Falcato >Reviewed-by: Gregory Price (Meta) >Tested-by: Gregory Price (Meta) >Tested-by: Muhammad Usama Anjum >Signed-off-by: David Hildenbrand (Arm) >--- > mm/khugepaged.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c >index 34ebb37c862c0..ce794d9760a87 100644 >--- a/mm/khugepaged.c >+++ b/mm/khugepaged.c >@@ -1995,9 +1995,10 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign > /* > * Must clear entry, or a racing truncate may re-remove it. > * TLB flush can be left until pmdp_collapse_flush() does it. >- * PTE dirty? Shmem page is already dirty; file is read-only. > */ >- clear_ptes(mm, addr, pte, nr_batch_ptes); >+ ptent = get_and_clear_ptes(mm, addr, pte, nr_batch_ptes); I went through get_and_clear_ptes() as well. It merges dirty bits across the batch, so we're covered even when the first PTE is clean and a later one is dirty. LGTM! Reviewed-by: Lance Yang >+ if (pte_dirty(ptent)) >+ folio_mark_dirty(folio); > folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma); > nr_mapped_ptes += nr_batch_ptes; > } > >-- >2.43.0 > >