Re: [PATCH v4 0/4] mm/truncate: fix data loss when truncating straddling large folios

[Brian Foster <bfoster@redhat.com>]

On Tue, Sep 22, 2026 at 07:06:59PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> 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 <bfoster@redhat.com>

Brian

> [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
>