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Mon, 26 Oct 2026 13:26:11 -0400 (EDT) Date: Mon, 26 Oct 2026 17:26:10 +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, stable@vger.kernel.org Subject: Re: [PATCH 11/16] mm/shmem: rescan after nonuniform folio splitting Message-ID: References: <20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org> <20261026-fixes_shmem-v1-11-703206d5fa0c@kernel.org> X-Mailing-List: stable@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: <20261026-fixes_shmem-v1-11-703206d5fa0c@kernel.org> On Mon, Oct 26, 2026 at 10:40:44AM +0100, David Hildenbrand (Arm) wrote: > The final scan in shmem_undo_range() restarts after a successful split > only if the original folio is no longer large. A nonuniform split can > leave that folio large, so the scan skips the newly created folios. > Hole punching then leaves pages within the hole allocated even though > they could be discarded. > > For example, a fault below the hole can allocate a 2 MiB folio after > boundary handling. Punching a 1 MiB hole at offset 64 KiB splits the > folio successfully but leaves the original folio at order 4. The scan > continues past all 512 pages, leaving 2 MiB allocated instead of 1 MiB. > The hole reads as zero, but the pages and their block charges remain. > > Restart when the original folio is still in the mapping after partial > truncation, identified by checking folio->mapping. If the helper > removed the whole folio, no rescan is needed. > > Fixes: 7460b470a131f ("mm/truncate: use folio_split() in truncate operation") > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) Acked-by: Kiryl Shutsemau (Meta) Do we want split state after truncate_inode_partial_folio() to be communicated more explicit? ->mapping check works, but it might not be obvious why. Just an idea for a follow up. -- Kiryl Shutsemau / Kirill A. Shutemov