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b=BotLVmlzy4TZljtLczP71diLAjatbqzAQEkLZp7bs9LTerIp5Qui3KjuhQeeT4l8y g6gDADj0uuPyyBR3/gn45tABr4F+L9DEc7hoSpkBt8FAsqG65DNR+ExxaJwzfI+27O 6A6t9+eSkEklG4w4NGD9aGjIUv8fmEET9xQq5AACqjr30EQsKl8n6M6dSFYBD8/gME SAFhyJNtMsoOl29zYLOYHf4fLZY3ulEkDQAUjD+pvQI17pWO/bTChoQvsRdHyRja5K OWIzo84jQYEScEB5hF+VocHW1TccpKKgvO2N+vIpntsTF00g/UjzLbZPiJyVQkkesr PQdk+5cMK7eSw== Message-ID: <699d2ada-6407-4758-82ad-927426ee049e@kernel.org> Date: Tue, 27 Oct 2026 11:07:48 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 09/16] mm/shmem: preserve data in clean folios when splitting fails To: Kiryl Shutsemau 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 References: <20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org> <20261026-fixes_shmem-v1-9-703206d5fa0c@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 3904940002 X-Stat-Signature: iwnnst4qqf3dx9cdgb5xaaaf6wmpxbxk X-Spam-Status: No, score=-7.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1793097825-427438 X-HE-Meta: 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 pRMsrqYI ta3Nyv8fc6EK+TkPk4lkaWDm3s/kbyEHgPHZyDVuOiL+CKk5++pGc6sJn3+QJxR4RTXL2ZUILtFvOELmaWa/uJqmRK15FGdPTNsgvklGesYYehSBtLSavgGZA6VIQBVaPg3lauzsA8kbwkzyC62RNQ+i6jralZwjQ3DWqz/6Cp2LwDZ48XjJUfNi4ctI9UbfgUs0e3oRwIHLx+cN9dmpfNS9cOPINgBFOe7BLoMiKtDyspsgRGqdUdgr4tP2uk4HT6yWbjRBVjZBieD1nUoIKzPn5xXiqVZv8SoHcO8RfZILHt5juvI5PQgRh8N11qhygDkyVyc8RX+i9vsEnu6AXOJ485/d52j3WAGEynqTUGniD+yliR9RfeNFKYWhjobhGsL7g 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 10/26/26 18:12, Kiryl Shutsemau wrote: > 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 > I'll add it, thanks! -- Cheers, David