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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-Archived-By: 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 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 6BC80180006 X-Stat-Signature: azjadr49nwpa68rzocgu78wdt8f9f6x8 X-Rspam-User: X-Spam-Status: No, score=-17.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793096035-309735 X-HE-Meta: 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 ZB+LCZ0j vCSm3VgNxUjiFQYV6F1dlrSe9rlTTFZsXUmJYPS6oBgRnAx/9Slh1pvugcL0oP+5j1e+KV4BCip4k5IUCer2BQJ+7BOBrXIdoyrJNmrgk5TsdrHhCPCuN7RjEJRIQizDfCIf9UvP+DXdBfiytQozV7gH/1aJUo+bmg82F/Qp2o+ekr1Nn97QGzM5y74Ux/xsw1s8nvp060iylTVG6nhL88P57clDJZZ+CMt4UraU0b1GTJ8xfqgTNXS9HcalB9BV1soLUhbE4MoUQ8xVUDyQWtK0xTxlGbGe7epB2r6TxzDQpNIb4/FycE8uMR+32SUDa5mpjhTC25dGFx0nk6Sdfq07bBPoGDDeOTykKtjm9GK6QB+Q= 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 10/27/26 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