Re: [PATCH v3 1/3] mm/truncate: fix data loss when splitting straddling large folios fails
From: Zhang Yi
Date: Thu Sep 17 2026 - 07:46:52 EST
On 9/17/2026 2:03 AM, Zi Yan wrote:
On Wed Sep 16, 2026 at 8:02 AM EDT, Jan Kara wrote:
On Wed 16-09-26 17:24:48, 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() -> 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 the page range to discard:
- Add pgoff_t *pstart and *pend out-parameters that receive the page
range fully covered by [lstart, lend] after any split (or none),
i.e. the pages wholly within the range and safe to discard.
- Adjust the ordering of the validate check when splitting folio2.
folio2->index is only reliable after the reference count and lock
have been successfully acquired, since it may have been split
concurrently, or freed and recycled to an unrelated mapping. On any
failure to obtain a reliable end position, fall back to
folio->index, which is safe but leaves the sub-folios split off at
the offset edge in the page cache.
- 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>
The changes mostly look good to me but I have some confusion around the
folio2 splitting below.
@@ -259,32 +271,62 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
* for shmem truncate
*/
struct folio *folio2;
+ pgoff_t end, aligned_end = (pos + offset + length) >>
+ PAGE_SHIFT;
- if (offset + length == size)
- goto no_split;
+ if (pstart)
+ *pstart = round_up(pos + offset, PAGE_SIZE) >>
+ PAGE_SHIFT;
+
+ if (offset + length == size) {
+ end = aligned_end;
+ goto out;
+ }
split_at2 = folio_page(folio,
PAGE_ALIGN_DOWN(offset + length) / PAGE_SIZE);
folio2 = page_folio(split_at2);
+ /*
+ * folio2 may become stale due to a concurrent split or
+ * freeing, so validate it before and after taking its lock.
+ * If it fails, we can't get an accurate end position and fall
+ * back to folio->index, which may leave sub-folios split off
+ * at the offset edge in the page cache this round.
+ */
+ end = folio->index;
if (!folio_try_get(folio2))
- goto no_split;
-
- if (!folio_test_large(folio2))
goto out;
+ if (folio2->mapping != folio->mapping ||
+ !folio_test_large(folio2))
+ goto out_put;
+
if (!folio_trylock(folio2))
- goto out;
+ goto out_put;
- /* 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);
+ if (page_folio(split_at2) != folio2) {
+ folio_unlock(folio2);
+ goto out_put;
+ }
+ if (!folio_test_large(folio2)) {
+ end = aligned_end;
+ folio_unlock(folio2);
+ goto out_put;
+ }
So I always found this folio2 lookup and revalidation somewhat suspicious
and now that we're digging into it I'll ask: As you note in the changelog,
by the time we compute split_at2 the page can be already freed and reused
because it was split off from the original 'folio'. It can be for example a
slab page or anything else. So is it guaranteed that page_folio() actually
For a slab or unrelated page, folio->mapping check rejects them.
returns something sensible? What guarantees we properly detect the "reuse
No, but folio_try_get() prevents non ref'd folios.
for something else" case in all possible cases for which the page can be
reused? I understand this is mostly a preexisting issue so maybe these
questions are more for MM guys than you...
I agree with you that trying to split a unlocked and not ref'd folio2 is
flaky. And it almost does a __filemap_get_folio() like you proposed
below.
I think this is indeed a pre-existing issue, as Sashiko pointed out:
> [Severity: Critical]
> Does this speculative lookup handle the reallocation race correctly if the
> folio is reallocated to a different file offset?
>
> After the first split, the tail pages are added to the page cache and
> unlocked. During speculative lookup, the tail page could be concurrently
> reclaimed, freed, and reallocated as a new large folio in the same mapping
> but at a completely different file offset.
>
> The validation checks earlier and the page_folio check here will all pass
> because the physical page split_at2 is part of the new allocation.
>
> However, the file index of the folio is not verified. If this unrelated large
> folio fails to split, the fallback path assigns its index to end here:
>
> if (folio_split_or_unmap(folio2, split_at2, min_order))
> end = folio2->index;
>
> This unrelated file offset is then returned to the caller via pend, which
> can cause the hole punch loop to truncate an unrelated segment of the file.
>
> Should we verify that the file index of folio2 matches the expected index
> (e.g. folio2->index + folio_page_idx(folio2, split_at2) == expected_index)
> before using it?
the current mapping check on the folio2 and the folio !=
page_folio(split_at) check in __folio_split() are not able to catch this
problem, which could then cause folio2 to be split at the wrong
position. Does that sound right?
So for me as an filesystem guy I'd appreciate some comment in this code
explaining why grabbing folio2 this way is actually safe. The really safe
way of getting to folio2 would be to use
__filemap_get_folio(mapping, (offset+length) >> PAGE_SHIFT, FGP_LOCK, 0)
but I suppose we don't use the mapping lookup as it is more expensive?
__filemap_get_folio() is a better version of the existing folio2 finding
code. When I wrote the code, I never thought about using
__filemap_get_folio(), but I probably should have done it.
BTW,
__filemap_get_folio(mapping, folio->index + (offset+length) >> PAGE_SHIFT,
FGP_LOCK | FGP_NOWAIT, 0)
might be better:
1. folio->index is needed to get the right folio2,
2. nowait can us nonblocking.
Following your suggestion, I reworked my existing patch to use
__filemap_get_folio(), and it ends up looking roughly like the
following(not tested yet). What do you think?
Besides, if the issue Sashiko pointed out does indeed exist, I can
extract the __filemap_get_folio() change into a separate patch to fix it
on its own.
Thanks,
Yi.
diff --git a/mm/truncate.c b/mm/truncate.c
index 23c90f00b530..b5e5bdf5f9ef 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -279,51 +279,46 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart,
*pstart = round_up(pos + offset,
min_nrbytes) >> PAGE_SHIFT;
- if (offset + length == size) {
- end = aligned_end;
+ end = aligned_end;
+ if (offset + length == size)
goto out;
- }
-
- split_at2 = folio_page(folio,
- PAGE_ALIGN_DOWN(offset + length) / PAGE_SIZE);
- folio2 = page_folio(split_at2);
/*
- * folio2 may become stale due to a concurrent split or
- * freeing, so validate it before and after taking its lock.
- * If it fails, we can't get an accurate end position and fall
- * back to folio->index, which may leave sub-folios split off
- * at the offset edge in the page cache this round.
+ * After the first split at the start edge, the folio at the
+ * end edge may be freed and reused concurrently.
+ * __filemap_get_folio() looks up the straddler at
+ * aligned_end and returns it locked and ref'd with the
+ * mapping validated.
*/
- end = folio->index;
- if (!folio_try_get(folio2))
+ folio2 = __filemap_get_folio(folio->mapping, aligned_end,
+ FGP_LOCK | FGP_NOWAIT, 0);
+ if (IS_ERR(folio2)) {
+ /*
+ * No sub-folio straddles the boundary when aligned_end
+ * is empty so discarding up to it is safe. Otherwise
+ * the straddler is locked by someone else and we
+ * cannot obtain a reliable end position, so we fall
+ * back to folio->index. This is safe but may leave
+ * pages split off at the offset edge lingering in the
+ * page cache this round.
+ */
+ if (PTR_ERR(folio2) != -ENOENT)
+ end = folio->index;
goto out;
-
- if (folio2->mapping != folio->mapping ||
- !folio_test_large(folio2))
- goto out_put;
-
- if (!folio_trylock(folio2))
- goto out_put;
-
- if (page_folio(split_at2) != folio2) {
- folio_unlock(folio2);
- goto out_put;
- }
- if (!folio_test_large(folio2)) {
- end = aligned_end;
- folio_unlock(folio2);
- goto out_put;
}
- /* Split failed: back off to the head of the straddler */
+ /* Already at the minimum order, nothing to split */
+ if (folio_order(folio2) == min_order)
+ goto out_put;
+
+ split_at2 = folio_page(folio2, (aligned_end - folio2->index));
+
+ /* Split failed, keep the straddler intact */
if (folio_split_or_unmap(folio2, split_at2, min_order))
end = folio2->index;
- else
- end = aligned_end;
- folio_unlock(folio2);
out_put:
+ folio_unlock(folio2);
folio_put(folio2);
out:
if (pend)