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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=UwnkNvoo; 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 283C3606A31A; Tue, 27 Oct 2026 10:13:53 +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=UwnkNvoo Received: by smtp.kernel.org (Postfix) with ESMTPSA id E2B261F01825; Tue, 27 Oct 2026 10:13:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793096033; bh=VcjnV7QS2G+aidrtUKihb76qyh+JC3b0hokomy9CWtI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UwnkNvoos1ek2AkU4HMF6xMMIsDgOXZGtVXxowwR/TRjQw2QGKWFVBS9ZULzArNQM vKjbhEVbAE3XPTSOdy8+zQwqfOB/4dPDOtAjgwfFuR+DqJwgHcwlxraumzkxaHA2ib Va2JKy62cNXxS5F+0zUgxeVYwzpiDA2KcmRzwRB7TU2ZGdFjSlpkYuldKxpsncjev3 VX99o6PmXSBPUsXfoGoG2H1a1zhRt2weS+QJK9agGdCjnbHxq7Z4JjPoHfP4VxU7MW OUTqfd8SpKL3+WjGAgmochCxIqilm+wXLSmTgUwZQGbAORhAlqKbDbjxETH1txy4HI +owiB3GFSMiyg== Message-ID: Date: Tue, 27 Oct 2026 11:03:24 +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 08/16] mm/shmem: stop retrying folios that cannot be split To: Zi Yan , Andrew Morton , Gregory Price , 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 Cc: 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-v1-8-703206d5fa0c@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: 7bit On 10/27/26 02:34, Zi Yan wrote: > On Mon Oct 26, 2026 at 5:40 AM EDT, David Hildenbrand (Arm) wrote: >> From: Chris Mason >> >> A fault or collapse can insert a large folio crossing EOF after >> shmem_undo_range() has handled the boundary folios. If splitting that >> folio fails, truncate_inode_partial_folio() retains it when it is dirty. >> The final scan then repeatedly finds it because it restarts from an >> unchanged start. >> >> Truncation keeps the inode lock and ignores fatal signals while looping. >> A concurrent unlink() can then block other directory operations while >> holding the directory lock and waiting for the file's lock. >> >> Advance start past the retained folio, whose truncated part has already >> been zeroed. Do not move start backwards if the failed split has shrunk >> the folio. >> >> Fix generated by an LLM and sent by Chris Mason in private. >> >> Fixes: 55ac8bbe358b ("mm/shmem: fix race in shmem_undo_range w/THP") >> Cc: stable@vger.kernel.org >> Signed-off-by: Chris Mason >> [ david: split fixes, simplify descriptions, adjust comments ] >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/shmem.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> > > I rebased your patch on mm-stable, which has Zhang Yi's fix, and > 1. tweaked the comment, > 2. skipped the dropped folio (this is to avoid a narrow case below). > > Since after Zhang Yi's patchset, truncate_inode_partial_folio() is > changed to return false when it drops the entire folio. Restetting > start for a dropped folio in the scenario below would leave zeroed > folios still in the memory, causing a waste, although it is very narrow: > > folio A=[0, 512), B=[512, 1024), C=[1024, 1536], > > 1. A, B and, C are inserted after the same_folio block check, so only the > code at whole_folios label sees all of them, > 2. A is split partially and the split failed, A becomes [0, 32) and > [32, 512) zeroed, > truncate_inode_partial_folio() returns false, start is set to 32, > 3. B is dropped entirely, false is returned, start is set to 1024, > 4. C is split successfully > > After 4, index is start=1024. The [32, 512) folio is not dropped. That > is why I added the folio->mapping check to avoid changing start in 3. > > > > diff --git a/mm/shmem.c b/mm/shmem.c > index fadf23f9e21ed..efd54a624974c 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -1480,6 +1480,16 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, > index = start; > break; > } > + } else { > + /* > + * Splitting failed, but the part in the > + * range is already zeroed. If the folio > + * was not dropped, skip it when > + * restarting the scan to avoid retrying > + * it forever. > + */ > + if (folio->mapping) > + start = max(start, folio_next_index(folio)); > } > } > folio_unlock(folio); > > Thanks, makes my life a lot easier! I'll change that to an effective: diff --git a/mm/shmem.c b/mm/shmem.c index 1a81f6dca7510..c263f02fe07cf 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1481,6 +1481,15 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, index = start; break; } + } else if (folio->mapping) { + /* + * Splitting failed, but the part in the + * range is already zeroed. The folio + * was not dropped, so skip it when + * restarting the scan to avoid retrying + * it forever. + */ + start = max(start, folio_next_index(folio)); } } -- Cheers, David