[PATCH 11/16] mm/shmem: rescan after nonuniform folio splitting

["David Hildenbrand (Arm)" <david@kernel.org>]

The final scan in shmem_undo_range() restarts after a successful split
only if the original folio is no longer large. A nonuniform split can
leave that folio large, so the scan skips the newly created folios.
Hole punching then leaves pages within the hole allocated even though
they could be discarded.

For example, a fault below the hole can allocate a 2 MiB folio after
boundary handling. Punching a 1 MiB hole at offset 64 KiB splits the
folio successfully but leaves the original folio at order 4. The scan
continues past all 512 pages, leaving 2 MiB allocated instead of 1 MiB.
The hole reads as zero, but the pages and their block charges remain.

Restart when the original folio is still in the mapping after partial
truncation, identified by checking folio->mapping. If the helper
removed the whole folio, no rescan is needed.

Fixes: 7460b470a131f ("mm/truncate: use folio_split() in truncate operation")
Cc: stable@vger.kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index c8608026e06ef..fcec40a2dd707 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1487,14 +1487,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 					truncate_inode_folio(mapping, folio);
 				} else if (truncate_inode_partial_folio(folio, lstart, lend)) {
 					/*
-					 * If we split a page, reset the loop so
-					 * that we pick up the new sub pages.
-					 * Otherwise the THP was entirely
-					 * dropped or the target range was
-					 * zeroed, so just continue the loop as
-					 * is.
+					 * If the folio was not removed, it
+					 * was split. Retry the scan to find
+					 * the split folios.
 					 */
-					if (!folio_test_large(folio)) {
+					if (folio->mapping) {
 						folio_unlock(folio);
 						index = start;
 						break;

-- 
2.43.0