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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=c5uSrUhY; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D2C94606A31F; Tue, 27 Oct 2026 10:43:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=c5uSrUhY Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D1C11F031E0; Tue, 27 Oct 2026 10:43:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793097837; bh=RUGcoGJ3mRnt/Z7GBxa2RJO4TTMxywHgVzZ1aSTOpqE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=c5uSrUhYlCsGKMO8Vp8lcJXtrvW1Yd9wQv6SBFXz1V0+3wtAnO8fPiQP0N44b5Caw JEfPDn2OgjccvQPVnX9M/V4VDQdTor9tUu92/PRDa9si9aYVzDcMuUTkZ63f/xl+MY wZIC3SiiuburrWWPpelVf2/XqjSKfYfDyW5QyoJ6TzsM0fdTaa/1gMoPCO+kaODY6M gteuPkd5rNdSnaN5sCYooq12TpUeft37PUUaXuIyhur4BrRBlfcjiRhAucIDo0OU2g F4JJK0ITfJexW69BQBa2a8huNvhSK4zOzUugogaUwEtLU2KWCRkeE52k0Z4b/7BDQX yLto4UgWV6vUQ== Message-ID: <1748ab4b-c219-4eb2-9f0b-cc540c7986f4@kernel.org> Date: Tue, 27 Oct 2026 11:14:09 +0100 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 12/16] mm/shmem: fix truncate hang in large swap entry To: Baolin Wang , Andrew Morton , Gregory Price , Zi Yan , Joshua Hahn , Rakie Kim , Ying Huang , Johannes Weiner , Vladimir Davydov , Michal Hocko , Hugh Dickins , 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org> <20261026-fixes_shmem-v1-12-703206d5fa0c@kernel.org> <152d698f-d7bd-4e1a-9736-412d52e2370c@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/27/26 09:59, Baolin Wang wrote: > > > On 10/26/26 5:40 PM, David Hildenbrand (Arm) wrote: >> Truncating a tmpfs file or memfd to a page-aligned size inside a large >> swap entry can loop indefinitely in shmem_undo_range(). The task holds >> the inode lock and cannot be killed. >> >> A large swap entry represents a whole swapped-out folio. For example, >> truncate a file whose pages 0-7 share one swap entry to four pages: >> >>      [ 0  1  2  3 | 4  5  6  7 ] >>           keep    |   discard >>                   ^ >>              new EOF, start = 4 >> >> i_size has already been updated when boundary handling asks >> shmem_get_partial_folio() for page 4. Since it is swapped out, the helper >> calls shmem_get_folio() with SGP_READ. That rejects page 4 with -EINVAL >> because its starting offset equals i_size. Nothing is read back. >> >> The final scan cannot free the entry without losing pages 0-3. It skips >> it, restarts from start = 4, and finds it again indefinitely. An unaligned >> truncate avoids this particular failure because the requested page starts >> below i_size. Folios already in the page-cache cannot trigger it as the >> direct lookup will return them beyond EOF. >> >> Fix it by letting shmem_get_partial_folio() call shmem_swapin_folio() >> directly for swap entries. This bypasses the read path's i_size check, >> allowing the existing boundary handling to zero and split the folio. If >> swap-in returns a smaller folio, it also splits the swap entry so the final >> scan can free the entries beyond EOF. Retry the lookup if the entry >> changes during swap-in. > > Make sense to me. > >> Handling swap-in allocation failures remains broken and is unchanged by >> this commit. Even when shmem_swapin_folio() correctly returns -ENOMEM >> instead of -EEXIST, shmem_get_partial_folio() returns NULL and >> shmem_undo_range() skips boundary handling. If the large swap entry >> remains intact, truncation can still loop in the final scan, while hole >> punching can return without zeroing the boundary. >> >> This may read an entire large folio back from swap while holding >> i_rwsem, only to discard part of it. Avoiding that I/O would require >> splitting both the swap entry and any large folio still in the swap >> cache. That, however, is much bigger work. > > But I think this case is extremely rare, where every attempt to call > shmem_split_large_entry() to split the large swap entry in the loop fails. At > that point, the system is already in serious trouble, where even OOM kill cannot > free up memory for the xarray allocation. Hard to believe this can actually > happen in practice:). So, it's fairly easy to reproduce with memcgs. But yeah, I assume this is a corner case. (hopefully :D ) -- Cheers, David