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Wed, 23 Sep 2026 06:32:16 -0700 (PDT) Message-ID: <4277af28-222b-49de-9372-4087f1a7e942@gmail.com> Date: Wed, 23 Sep 2026 21:31:46 +0800 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/4] mm/truncate: fix data loss when truncating straddling large folios To: Brian Foster , Zhang Yi Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, hughd@google.com, baolin.wang@linux.alibaba.com, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, joannelkoong@gmail.com, djwong@kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, chengzhihao1@huawei.com, wangkefeng.wang@huawei.com, yukuai@fnnas.com References: <20260922110703.468389-1-yi.zhang@huaweicloud.com> Content-Language: en-US From: Zhang Yi In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/22/2026 9:47 PM, Brian Foster wrote: > On Tue, Sep 22, 2026 at 07:06:59PM +0800, Zhang Yi wrote: >> From: Zhang Yi >> >> Hello, >> >> This is the fourth version fixing data loss when truncating straddling >> large folios caught on the upcomming ext4 + iomap buffered I/O >> conversion. >> >> When truncate_inode_pages_range() punches a hole or truncates a file, >> truncate_inode_partial_folio() splits a large folio so that the caller >> can drop the in-range sub-folios while keeping the out-of-range tail >> intact. This series fixes three distinct problems in that path that >> can each lose the valid out-of-range tail of a straddling folio, plus a >> follow-up that clarifies the return value semantics. >> >> Patch 01 aligns the truncation boundaries inwards to the mapping minimum >> folio order in truncate_inode_pages_range(). With a non-zero min_order, >> folio_split() stops at min_order instead of order 0, so a boundary >> computed at page granularity can land inside a min-order-aligned >> sub-folio and the truncate loop drops that whole chunk, valid tail >> included, causing data loss. >> >> Patch 02 looks the end-edge straddler up by its page index through >> __filemap_get_folio() in truncate_inode_partial_folio(). After the >> first split the straddler is unlocked and only transiently ref'd in the >> page cache, so the page pointer derived from the original folio can be >> freed and reallocated as a different folio in the same mapping, and the >> mapping check cannot catch it, which may cause incorrect splitting and >> potential data loss. >> >> Patch 03 reworks the contract between truncate_inode_partial_folio() and >> its callers. If the second split of the straddler fails, the function >> reported success unconditionally, and the leftover incorrect end >> position could cause the truncate loop to drop that valid tail. After >> rework, it tells the caller the exact page range safe to discard via new >> pstart/pend out-parameters, so the truncate loop never touches a >> straddling folio that still holds valid out-of-range data. >> >> Patch 04 clarifies the return value semantics to "at least one split >> succeeded", which is all the shmem caller needs to decide whether to >> reset its scan loop. >> >> >> The second patch fixes a pre-existing race issue that is reachable >> today, so it is Cc'd to stable. Patches 01 and 03 require a dirty large >> folio that carries no filesystem private data, so they are not reachable >> on current filesystems. They were found while developing the upcoming >> ext4 iomap buffered I/O path[1]. >> >> Thanks, >> Yi. >> > > Hi Yi, > > Modulo Jan's comment on patch 4, this series looks good to me. FWIW: > > Reviewed-by: Brian Foster > > Brian Thank you for the review! Yi. > >> [1] https://lore.kernel.org/linux-fsdevel/a638a8fb-c184-4069-ae33-379ec12cd514@huaweicloud.com/ >> >> >> v3->v4: >> - Move the patch that fixes data loss when min_order is non-zero to the >> first patch position, aligning start and end in >> truncate_inode_pages_range(). (Zi Yan) >> - Add patch 2, fixing the invalid folio2 issue under concurrency when >> truncate_inode_partial_folio() splits at the end position. Use >> __filemap_get_folio() to obtain a reliable folio2. (Jan Kara) >> >> v2->v3: >> - Rework the folio2 validity check logic to fix the invalid >> folio->index issue. (sashiko) >> - Clarify the pstart and pend setting logic and the corresponding >> comments to make it more readable. (Brian, Joanne) >> - Split the patch into 3 small patches. (Zi Yan) >> >> v1->v2: >> - Export pstart as a new parameter so that the generic and shmem >> truncate paths don't need to recompute the start value from the >> return value. (Brian) >> - When min_order is non-zero, align [pstart, pend] to the inner >> boundaries of the folio to ensure they do not point into the middle >> of a large folio, which could otherwise cause valid data within the >> folio to be incorrectly cleared. (Joanne) >> >> v3: https://lore.kernel.org/linux-mm/20260916092450.654408-1-yi.zhang@huaweicloud.com/ >> v2: https://lore.kernel.org/linux-mm/20260909062339.473816-1-yi.zhang@huaweicloud.com/ >> v1: https://lore.kernel.org/linux-mm/20260903115018.2034541-1-yi.zhang@huaweicloud.com/ >> >> >> Zhang Yi (4): >> mm/truncate: align truncation boundaries to mapping minimum folio >> order >> mm/truncate: look up the end-edge straddler by index >> mm/truncate: fix data loss when splitting straddling large folios >> fails >> mm/truncate: clarify return value of truncate_inode_partial_folio() >> >> mm/internal.h | 4 +- >> mm/shmem.c | 13 ++--- >> mm/truncate.c | 128 +++++++++++++++++++++++++++++++++----------------- >> 3 files changed, 91 insertions(+), 54 deletions(-) >> >> -- >> 2.54.0 >> >