Re: [PATCH v4 3/4] mm/truncate: fix data loss when splitting straddling large folios fails

[Zi Yan]

On Tue Sep 22, 2026 at 7:07 AM EDT, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>
> truncate_inode_partial_folio() splits a large folio so that the caller's
> truncate loop can drop the in-range sub-folios while keeping the
> out-of-range tail. The first split at the punch start edge is
> non-uniform, which leaves the sub-folio at the truncation end edge as
> large as possible, this means it may still straddle the range, holding
> both zeroed in-range and valid out-of-range data. The function then
> attempts a second split at offset + length to isolate that tail.
>
> If the second split fails the straddling sub-folio stays merged. The
> function returned true unconditionally on all exit paths of the success
> block, telling the caller it was fully handled. The caller kept its
> default end and the truncate loop truncated every sub-folio below it,
> including the merged straddler, discarding the valid out-of-range tail.
>
> For example, a 4-page order-2 folio punched from offset 0 to the middle
> of the last page:
>
>   truncate_inode_pages_range()
>     truncate_inode_partial_folio()      # same_folio == true
>       1st split at page0 -> [p0, p1, p2-3]   # non-uniform, success
>       folio2 = p2-3 # straddles: p2 zeroed, p3 tail valid
>       2nd split of folio2 fails / cannot lock
>       return true                       # BUG: caller keeps default end
>     end = 3
>     loop truncates p0, p1, p2-3        # p3's valid tail is lost
>
> This became reachable after commit 7460b470a131 ("mm/truncate: use
> folio_split() in truncate operation") replaced the atomic split_folio()
> with folio_split(), whose non-uniform split can partially split a folio
> and leave the end edge merged.
>
> It has gone unnoticed because a dirty large folio normally carries the
> filesystem's private data, for example buffer_head, so
> filemap_release_folio() fails on a dirty folio and folio_split() aborts
> with -EBUSY before any split, leaving the straddler safely unsplit. The
> bug is only reachable on paths that produce dirty large folios without
> filesystem private data, and it was caught on the upcoming ext4 iomap
> buffered I/O path when no ifs is attached.
>
> Rework the contract so the caller is told the folio range to discard:
>
>   - Add pgoff_t *pstart and *pend out-parameters that receive the folio
>     range fully covered by [lstart, lend] after any split (or none),
>     aligned inwards to min_order, i.e. the folios wholly within the
>     range and safe to discard.
>
>   - Report a reliable end position to the caller. The straddler is
>     looked up at an index aligned inwards to the mapping minimum folio
>     order, and *pend is set to that boundary on success. If nothing
>     covers the boundary, discarding up to it stays safe. If the
>     straddler is locked by someone else, fall back to folio->index.
>     This best-effort fallback may leave the in-range sub-folios to a
>     later pass but never discards the out-of-range tail. If the
>     straddler cannot be split, fall back to folio2->index so the caller
>     keeps the out-of-range tail.
>
>   - Rename the byte-range parameters start/end to lstart/lend to better
>     express their semantics.
>
> Callers in truncate_inode_pages_range() and shmem_undo_range() pass
> &pstart for the folio at the start edge and &pend for the folio at the
> end edge, so the truncate loop drops exactly the fully covered pages and
> never touches a straddling folio that still holds valid out-of-range
> data.
>
> Suggested-by: Brian Foster <bfoster@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-fsdevel/anH-WKA1coW6wtfG@bfoster/
> Fixes: 7460b470a131 ("mm/truncate: use folio_split() in truncate operation")
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> ---
>  mm/internal.h |  4 +--
>  mm/shmem.c    | 13 +++-----
>  mm/truncate.c | 88 +++++++++++++++++++++++++++++++++++----------------
>  3 files changed, 68 insertions(+), 37 deletions(-)

<snip>
>
> @@ -251,6 +264,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)

Add more context:

	if (!folio_test_large(folio))
>  		return true;
>  
>  	min_order = mapping_min_folio_order(folio->mapping);
> +	min_nrbytes = mapping_min_folio_nrbytes(folio->mapping);
>  	split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE);
>  	if (!folio_split_or_unmap(folio, split_at, min_order)) {
>  		/*
> @@ -259,34 +273,57 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>  		 * for shmem truncate
>  		 */
>  		struct folio *folio2;
> -		pgoff_t end_idx;
> +		pgoff_t end, aligned_end = round_down(pos + offset + length,
> +						min_nrbytes) >> PAGE_SHIFT;

<snip>

> +		/* Already at the minimum order, nothing to split */
> +		if (folio_order(folio2) == min_order)
> +			goto out_put;

In the above, folio_test_large() is used to determine whether folio
needs to be split or not, but here folio_order() == min_order is used.
Should the above "if (!folio_test_large(folio))" be changed to use
min_order check to match the check here?

-- 
Best Regards,
Yan, Zi