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Mon, 26 Oct 2026 13:12:10 -0400 (EDT) Date: Mon, 26 Oct 2026 17:12:09 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Gregory Price , Zi Yan , Joshua Hahn , Rakie Kim , Ying Huang , Johannes Weiner , Vladimir Davydov , Michal Hocko , Hugh Dickins , Baolin Wang , Lukas Czerner , Carlos Maiolino , Christian Brauner , Jan Kara , Daniel Verkamp , Jeff Xu , Muchun Song , Oscar Salvador , David Stevens , "Matthew Wilcox (Oracle)" , William Kucharski , Chris Li , Kairui Song , Nhat Pham , Miaohe Lin , Naoya Horiguchi , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason , stable@vger.kernel.org Subject: Re: [PATCH 09/16] mm/shmem: preserve data in clean folios when splitting fails Message-ID: References: <20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org> <20261026-fixes_shmem-v1-9-703206d5fa0c@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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20261026-fixes_shmem-v1-9-703206d5fa0c@kernel.org> On Mon, Oct 26, 2026 at 10:40:42AM +0100, David Hildenbrand (Arm) wrote: > From: Chris Mason > > A shared writable mapping can have dirty PTEs while its shmem folio is > still clean. A read fault can install a writable PTE, and a later store > sets the PTE's dirty bit without marking the folio dirty. > > After shmem_undo_range() has handled the boundary folios, a concurrent > read fault into a hole below the new EOF can allocate a large folio. > For example, on a tmpfs mount with huge=always, the fault's offset must > be within the file, but the allocated folio can extend beyond i_size. > The final scan then finds this folio and calls > truncate_inode_partial_folio() without marking it dirty. This is hard to follow. I asked LLM for a diagram. I think it would be helpful for the commit message: CPU0: ftruncate(fd, 3M) CPU1: MAP_SHARED, tmpfs huge=always ───────────────────────────────────── ────────────────────────────────── i_size = 3M shmem_undo_range(3M, -1) first pass: hole, nothing to do boundary folio at 3M: hole, none read fault at 2M+x (below EOF) PMD folio [2M,4M), crosses EOF writable PTE, folio clean store to 2M+x PTE dirty, folio still clean vmsplice(2M+x) extra folio reference whole_folios: finds [2M,4M) folio truncate_inode_partial_folio() zero [3M,4M) folio_split() unmap_folio(): extra ref → -EAGAIN, PTE dirty bit never reaches the folio folio clean → truncate_inode_folio(whole) [2M,3M) dropped read 2M+x → refault → zeroes > Splitting normally collects the PTE dirty bits when unmapping the > folio. An extra reference, for example from vmsplice(), can make it > fail before that step. The fallback unmapping on split failure excludes > shmem, so the helper still sees a clean folio and drops it, losing data > outside the range. > > Mark large folios dirty before partial truncation in the final scan, > as the boundary handling already does. Exclude failed-fallocate > rollback, which must discard the folios it added. > > Fix generated by an LLM and sent by Chris Mason in private. > > Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios") > Cc: stable@vger.kernel.org > Signed-off-by: Chris Mason > [ david: simplify descriptions, adjust comments ] > Signed-off-by: David Hildenbrand (Arm) The fix itself looks reasonable: Acked-by: Kiryl Shutsemau (Meta) -- Kiryl Shutsemau / Kirill A. Shutemov