Re: [PATCH v4 1/4] mm/truncate: align truncation boundaries to mapping minimum folio order

[Zi Yan]

On Tue Sep 22, 2026 at 7:07 AM EDT, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>
> When the mapping has a non-zero minimum folio order (min_order),
> folio_split() in truncate_inode_partial_folio() stops at min_order
> instead of order 0, so the sub-folio containing a split point stays
> aligned to 1 << min_order rather than to a single page. The original
> boundaries in truncate_inode_pages_range() were based on page
> granularity, so either boundary could land inside the min_order chunk at
> its edge, and the truncation loop would drop that whole chunk, valid
> out-of-range tail included.
>
> For example, a 64K (order-4) folio with min_order = 2 (16K) punched from
> offset 0 to 36K:
>
>   split @p0 -> [p0-p3, p4-p7, p8-p15]         # non-uniform, min_order
>   folio2 = p8-p15           # straddles: p8 in range, p9-p15 tail valid
>   2nd split of folio2 -> [p8-p11, p12-p15]    # success
>   end(old) = p9                               # BUG: p9 inside [p8-p11]
>   loop truncates ... p8-p11               # p9-p11's valid tail is lost
>
> It has gone unnoticed so far for two reasons. A non-zero min_order is
> only used by filesystems with a block or sector size larger than the
> page size, and those either always write back the affected range before
> punching a hole or truncating, or they carry filesystem private data on
> dirty folios (e.g. buffer_head), which makes filemap_release_folio()
> fail and folio_split() abort with -EBUSY, so the folio is never split
> and the old start/end boundaries remain valid. The bug only becomes
> reachable on paths that truncate dirty large folios without prior
> writeback and without filesystem private data, such as the upcoming ext4
> iomap buffered I/O path.
>
> Align both start (rounded up) and end (rounded down) to the mapping
> minimum folio order so they always fall on a folio boundary.
>
> Reported-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> Link: https://lore.kernel.org/linux-mm/CAJnrk1bQYUe6+1ryyJur5EEnZYrC+_5AYsy=OWzVRgD4202y1g@xxxxxxxxxxxxxx/
> Fixes: e220917fa5077 ("mm: split a folio in minimum folio order chunks")
> Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> ---
>  mm/truncate.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)

LGTM. Just a nit below.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>
> diff --git a/mm/truncate.c b/mm/truncate.c
> index b58ba940be47..f9625bb4916f 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -345,9 +345,11 @@ long mapping_evict_folio(struct address_space *mapping, struct folio *folio)
>   * @lstart: offset from which to truncate
>   * @lend: offset to which to truncate (inclusive)
>   *
> - * Truncate the page cache, removing the pages that are between
> - * specified offsets (and zeroing out partial pages
> - * if lstart or lend + 1 is not page aligned).
> + * Truncate the page cache, removing the folios that are between specified
> + * offsets (and zeroing out partial folios if lstart or lend + 1 is not
> + * folio aligned).  For mappings with a non-zero minimum folio order, the
> + * boundaries are aligned inwards to 1 << min_order so the edge sub-folio
> + * straddling the range is kept.
>   *
>   * Truncate takes two passes - the first pass is nonblocking.  It will not
>   * block on page locks and it will not block on writeback.  The second pass
> @@ -374,14 +376,14 @@ void truncate_inode_pages_range(struct address_space *mapping,
>  	int		i;
>  	struct folio	*folio;
>  	bool		same_folio;
> +	pgoff_t		min_nrpages = mapping_min_folio_nrpages(mapping);
>  

It is better to put it at the top (reverse christmas tree).


-- 
Best Regards,
Yan, Zi