[RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio()

[RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio()

[Zhang Yi]

From: Zhang Yi <yi.zhang@huawei.com>

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() -> iomap_release_folio() returns false 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 where to stop instead of
silently truncating the straddler:

  - Return true only when a split occurred, false otherwise. This
    clarifies the existing confusing return value semantics.

  - Add an optional out-parameter pgoff_t *end, set to the index of the
    folio that contains @lend and must be kept by the caller's loop. It
    is only written when the folio actually straddles @lend. On the
    success path it defaults to the page index of the end edge and is
    refined to folio2->index when the second split fails to isolate the
    tail.

  - Rename the byte-range parameters start/end to lstart/lend to avoid
    clashing with the new @end output and to separate byte offsets from
    folio indices.

Callers in truncate_inode_pages_range() and shmem_undo_range() pass &end
only when the folio straddles lend. After all, no caller discards a
straddling folio anymore, the in-range cleanly-split sub-folios below it
are still dropped.

Suggested-by: Brian Foster <bfoster@redhat.com>
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@huawei.com>
---
 mm/internal.h |  4 ++--
 mm/shmem.c    | 12 +++++------
 mm/truncate.c | 57 ++++++++++++++++++++++++++++++---------------------
 3 files changed, 41 insertions(+), 32 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 68db5abd0a4c..db7d9d9fb5c9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -627,8 +627,8 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
 unsigned find_get_entries(struct address_space *mapping, pgoff_t *start,
 		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
 int truncate_inode_folio(struct address_space *mapping, struct folio *folio);
-bool truncate_inode_partial_folio(struct folio *folio, loff_t start,
-		loff_t end);
+bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart,
+		loff_t lend, pgoff_t *end);
 long mapping_evict_folio(struct address_space *mapping, struct folio *folio);
 unsigned long mapping_try_invalidate(struct address_space *mapping,
 		pgoff_t start, pgoff_t end, unsigned long *nr_failed);
diff --git a/mm/shmem.c b/mm/shmem.c
index 89a1495e55f7..cc1548ff509a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1175,11 +1175,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 	if (folio) {
 		same_folio = lend < folio_next_pos(folio);
 		folio_mark_dirty(folio);
-		if (!truncate_inode_partial_folio(folio, lstart, lend)) {
+		if (!truncate_inode_partial_folio(folio, lstart, lend,
+						  same_folio ? &end : NULL))
 			start = folio_next_index(folio);
-			if (same_folio)
-				end = folio->index;
-		}
 		folio_unlock(folio);
 		folio_put(folio);
 		folio = NULL;
@@ -1189,8 +1187,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 		folio = shmem_get_partial_folio(inode, lend >> PAGE_SHIFT);
 	if (folio) {
 		folio_mark_dirty(folio);
-		if (!truncate_inode_partial_folio(folio, lstart, lend))
-			end = folio->index;
+		truncate_inode_partial_folio(folio, lstart, lend, &end);
 		folio_unlock(folio);
 		folio_put(folio);
 	}
@@ -1258,7 +1255,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 
 				if (!folio_test_large(folio)) {
 					truncate_inode_folio(mapping, folio);
-				} else if (truncate_inode_partial_folio(folio, lstart, lend)) {
+				} else if (truncate_inode_partial_folio(folio,
+							lstart, lend, NULL)) {
 					/*
 					 * If we split a page, reset the loop so
 					 * that we pick up the new sub pages.
diff --git a/mm/truncate.c b/mm/truncate.c
index b58ba940be47..2ebb00f6c379 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -206,15 +206,18 @@ static int folio_split_or_unmap(struct folio *folio, struct page *split_at,
 /*
  * Handle partial folios.  The folio may be entirely within the
  * range if a split has raced with us.  If not, we zero the part of the
- * folio that's within the [start, end] range, and then split the folio if
+ * folio that's within the [lstart, lend] range, and then split the folio if
  * it's large.  split_page_range() will discard pages which now lie beyond
  * i_size, and we rely on the caller to discard pages which lie within a
  * newly created hole.
  *
- * Returns false if splitting failed so the caller can avoid
- * discarding the entire folio which is stubbornly unsplit.
+ * When @end non-NULL, set to the index of the folio that contains @lend
+ * and must be kept by the caller's truncate loop.  Return %true if the
+ * folio was split, %false otherwise, in which case the folio is dropped or
+ * may still straddle the range, so the caller must not discard it.
  */
-bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
+bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart,
+				  loff_t lend, pgoff_t *end)
 {
 	loff_t pos = folio_pos(folio);
 	size_t size = folio_size(folio);
@@ -222,19 +225,22 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 	struct page *split_at, *split_at2;
 	unsigned int min_order;
 
-	if (pos < start)
-		offset = start - pos;
+	if (end && pos + size > (u64)lend)
+		*end = folio->index;
+
+	if (pos < lstart)
+		offset = lstart - pos;
 	else
 		offset = 0;
-	if (pos + size <= (u64)end)
+	if (pos + size <= (u64)lend)
 		length = size - offset;
 	else
-		length = end + 1 - pos - offset;
+		length = lend + 1 - pos - offset;
 
 	folio_wait_writeback(folio);
 	if (length == size) {
 		truncate_inode_folio(folio->mapping, folio);
-		return true;
+		return false;
 	}
 
 	/*
@@ -248,7 +254,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 	if (folio_needs_release(folio))
 		folio_invalidate(folio, offset, length);
 	if (!folio_test_large(folio))
-		return true;
+		return false;
 
 	min_order = mapping_min_folio_order(folio->mapping);
 	split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE);
@@ -259,6 +265,10 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 		 * for shmem truncate
 		 */
 		struct folio *folio2;
+		bool tail_isolated = true;
+
+		if (end)
+			*end = (pos + offset + length) >> PAGE_SHIFT;
 
 		if (offset + length == size)
 			goto no_split;
@@ -273,24 +283,28 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 		if (!folio_test_large(folio2))
 			goto out;
 
-		if (!folio_trylock(folio2))
+		if (!folio_trylock(folio2)) {
+			tail_isolated = false;
 			goto out;
+		}
 
 		/* make sure folio2 is large and does not change its mapping */
 		if (folio_test_large(folio2) &&
-		    folio2->mapping == folio->mapping)
-			folio_split_or_unmap(folio2, split_at2, min_order);
+		    folio2->mapping == folio->mapping &&
+		    folio_split_or_unmap(folio2, split_at2, min_order))
+			tail_isolated = false;
 
 		folio_unlock(folio2);
 out:
+		if (!tail_isolated && end)
+			*end = folio2->index;
 		folio_put(folio2);
 no_split:
 		return true;
 	}
-	if (folio_test_dirty(folio))
-		return false;
-	truncate_inode_folio(folio->mapping, folio);
-	return true;
+	if (!folio_test_dirty(folio))
+		truncate_inode_folio(folio->mapping, folio);
+	return false;
 }
 
 /*
@@ -413,11 +427,9 @@ void truncate_inode_pages_range(struct address_space *mapping,
 	folio = __filemap_get_folio(mapping, lstart >> PAGE_SHIFT, FGP_LOCK, 0);
 	if (!IS_ERR(folio)) {
 		same_folio = lend < folio_next_pos(folio);
-		if (!truncate_inode_partial_folio(folio, lstart, lend)) {
+		if (!truncate_inode_partial_folio(folio, lstart, lend,
+						  same_folio ? &end : NULL))
 			start = folio_next_index(folio);
-			if (same_folio)
-				end = folio->index;
-		}
 		folio_unlock(folio);
 		folio_put(folio);
 		folio = NULL;
@@ -427,8 +439,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
 		folio = __filemap_get_folio(mapping, lend >> PAGE_SHIFT,
 						FGP_LOCK, 0);
 		if (!IS_ERR(folio)) {
-			if (!truncate_inode_partial_folio(folio, lstart, lend))
-				end = folio->index;
+			truncate_inode_partial_folio(folio, lstart, lend, &end);
 			folio_unlock(folio);
 			folio_put(folio);
 		}
-- 
2.52.0

Re: [RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Missing re-validation of `page_folio()` result after `folio_try_get()` enables TOCTOU race, leading to out-of-bounds page splits and kernel panics.
--

commit 34ec9be8b97f90a8067df5949df00d3bb219ade3
Author: Zhang Yi <yi.zhang@huawei.com>

mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio()

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.

Link: https://lore.kernel.org/linux-fsdevel/anH-WKA1coW6wtfG@bfoster/

> diff --git a/mm/truncate.c b/mm/truncate.c
> index b58ba940be474..2ebb00f6c3793 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -273,24 +283,28 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>  		if (!folio_test_large(folio2))
>  			goto out;
>  
> -		if (!folio_trylock(folio2))
> +		if (!folio_trylock(folio2)) {
> +			tail_isolated = false;
>  			goto out;
> +		}
>  
>  		/* make sure folio2 is large and does not change its mapping */
>  		if (folio_test_large(folio2) &&
> -		    folio2->mapping == folio->mapping)
> -			folio_split_or_unmap(folio2, split_at2, min_order);
> +		    folio2->mapping == folio->mapping &&
> +		    folio_split_or_unmap(folio2, split_at2, min_order))
> +			tail_isolated = false;

[Severity: High]
Does this code have a TOCTOU race that could lead to an out-of-bounds page
split?

In truncate_inode_partial_folio() in mm/truncate.c, a speculative folio
lookup occurs just prior to this block:

	split_at2 = folio_page(folio,
			PAGE_ALIGN_DOWN(offset + length) / PAGE_SIZE);
	folio2 = page_folio(split_at2);

	if (!folio_try_get(folio2))
		goto no_split;

Without a re-validation check after folio_try_get(), couldn't a concurrent
operation (like memory reclaim or shmem readahead reallocation) change the
compound structure of folio2?

If the compound structure changes, a thread could successfully take a
reference on the new, smaller folio2. However, split_at2 would now be
outside the bounds of this new folio2.

When folio2 is later passed into folio_split_or_unmap() in the modified
code above, would this out-of-bounds split_at2 cause a kernel panic via
VM_BUG_ON_FOLIO() inside folio_split(), or corrupt adjacent page structures
if debug is disabled?

Should there be a check like if (unlikely(page_folio(split_at2) != folio2))
added after folio_try_get() to stabilize the reference?

>  
>  		folio_unlock(folio2);
>  out:

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260903115018.2034541-1-yi.zhang@huaweicloud.com?part=1

Re: [RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio()

[Joanne Koong]

On Thu, Sep 3, 2026 at 5:01 AM Zhang Yi <yi.zhang@huaweicloud.com> wrote:
>
> From: Zhang Yi <yi.zhang@huawei.com>
>
> 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() -> iomap_release_folio() returns false 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 where to stop instead of
> silently truncating the straddler:
>
>   - Return true only when a split occurred, false otherwise. This
>     clarifies the existing confusing return value semantics.
>
>   - Add an optional out-parameter pgoff_t *end, set to the index of the
>     folio that contains @lend and must be kept by the caller's loop. It
>     is only written when the folio actually straddles @lend. On the
>     success path it defaults to the page index of the end edge and is
>     refined to folio2->index when the second split fails to isolate the
>     tail.
>
>   - Rename the byte-range parameters start/end to lstart/lend to avoid
>     clashing with the new @end output and to separate byte offsets from
>     folio indices.
>
> Callers in truncate_inode_pages_range() and shmem_undo_range() pass &end
> only when the folio straddles lend. After all, no caller discards a
> straddling folio anymore, the in-range cleanly-split sub-folios below it
> are still dropped.
>
> Suggested-by: Brian Foster <bfoster@redhat.com>
> 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@huawei.com>
> ---
>  mm/internal.h |  4 ++--
>  mm/shmem.c    | 12 +++++------
>  mm/truncate.c | 57 ++++++++++++++++++++++++++++++---------------------
>  3 files changed, 41 insertions(+), 32 deletions(-)
>
> diff --git a/mm/truncate.c b/mm/truncate.c
> index b58ba940be47..2ebb00f6c379 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -206,15 +206,18 @@ static int folio_split_or_unmap(struct folio *folio, struct page *split_at,
>  /*
>   * Handle partial folios.  The folio may be entirely within the
>   * range if a split has raced with us.  If not, we zero the part of the
> - * folio that's within the [start, end] range, and then split the folio if
> + * folio that's within the [lstart, lend] range, and then split the folio if
>   * it's large.  split_page_range() will discard pages which now lie beyond
>   * i_size, and we rely on the caller to discard pages which lie within a
>   * newly created hole.
>   *
> - * Returns false if splitting failed so the caller can avoid
> - * discarding the entire folio which is stubbornly unsplit.
> + * When @end non-NULL, set to the index of the folio that contains @lend
> + * and must be kept by the caller's truncate loop.  Return %true if the
> + * folio was split, %false otherwise, in which case the folio is dropped or
> + * may still straddle the range, so the caller must not discard it.
>   */
> -bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
> +bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart,
> +                                 loff_t lend, pgoff_t *end)
>  {
>         loff_t pos = folio_pos(folio);
>         size_t size = folio_size(folio);
> @@ -222,19 +225,22 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>         struct page *split_at, *split_at2;
>         unsigned int min_order;
>
> -       if (pos < start)
> -               offset = start - pos;
> +       if (end && pos + size > (u64)lend)
> +               *end = folio->index;
> +
> +       if (pos < lstart)
> +               offset = lstart - pos;
>         else
>                 offset = 0;
> -       if (pos + size <= (u64)end)
> +       if (pos + size <= (u64)lend)
>                 length = size - offset;
>         else
> -               length = end + 1 - pos - offset;
> +               length = lend + 1 - pos - offset;
>
>         folio_wait_writeback(folio);
>         if (length == size) {
>                 truncate_inode_folio(folio->mapping, folio);
> -               return true;
> +               return false;
>         }
>
>         /*
> @@ -248,7 +254,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>         if (folio_needs_release(folio))
>                 folio_invalidate(folio, offset, length);
>         if (!folio_test_large(folio))
> -               return true;
> +               return false;
>
>         min_order = mapping_min_folio_order(folio->mapping);
>         split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE);
> @@ -259,6 +265,10 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>                  * for shmem truncate
>                  */
>                 struct folio *folio2;
> +               bool tail_isolated = true;
> +
> +               if (end)
> +                       *end = (pos + offset + length) >> PAGE_SHIFT;

If I'm understanding it correctly, based on how
truncate_inode_pages_range() uses the end value (eg the "while (index
< end)" loop condition and the find_get_entries(..., end - 1, ...)),
end needs to point to the start of the folio if the tail folio from
the split is a large folio, in order to exclude that folio from then
being truncated. But with the (pos + offset + length) >> PAGE_SHIFT
calculation here, does that result in some cases in it pointing to the
middle of a large folio? Maybe some logic is needed to make sure that
it points to the start?

Thanks,
Joanne