[PATCH] mm/shmem: drop the second folio batch from shmem_undo_range()

[Qiliang Yuan <qiliang.yuan@linux.dev>]

shmem_undo_range() keeps a folio_batch and an array of FOLIO_BATCH_SIZE
indices on its stack. Removing truncated folios from the page cache in
batches added a second folio_batch to collect the folios to delete.

That grows the stack frame by about 200 bytes, past the 1024 byte limit
of mips eyeq6_defconfig built with clang.

Pass fbatch itself to delete_from_page_cache_batch() once the swap
entries are gone, as truncate_inode_pages_range() does. Undoing a failed
fallocate() only removes the folios that are not uptodate, so remove
those one at a time with truncate_inode_folio() as before.

Stack frame of shmem_undo_range() built with clang 18:

  config                before (bytes)  after (bytes)  change
  mips eyeq6_defconfig             928            720    -22%
  x86_64                           920            712    -23%

Fixes: b97fbd541ecba ("mm/shmem: remove truncated folios from the page cache in batches")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202611112032.TMyPU6Rm-lkp@intel.com/
Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
David, this applies on top of b97fbd541ecba in shmem-7.5.misc; please
fold it in if you prefer.
---
 mm/shmem.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 156469423a866..5bcbdd6078a73 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1352,9 +1352,6 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 	index = start;
 	while (index < end && find_lock_entries(mapping, &index, end - 1,
 			&fbatch, indices)) {
-		struct folio_batch remove;
-
-		folio_batch_init(&remove);
 		for (i = 0; i < folio_batch_count(&fbatch); i++) {
 			folio = fbatch.folios[i];
 
@@ -1366,13 +1363,14 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				continue;
 			}
 
-			if (!unfalloc || !folio_test_uptodate(folio)) {
+			if (!unfalloc)
 				truncate_cleanup_folio(folio);
-				folio_batch_add(&remove, folio);
-			}
+			else if (!folio_test_uptodate(folio))
+				truncate_inode_folio(mapping, folio);
 		}
-		delete_from_page_cache_batch(mapping, &remove);
 		folio_batch_remove_exceptionals(&fbatch);
+		if (!unfalloc)
+			delete_from_page_cache_batch(mapping, &fbatch);
 		for (i = 0; i < folio_batch_count(&fbatch); i++)
 			folio_unlock(fbatch.folios[i]);
 		folio_batch_release(&fbatch);

---
base-commit: b97fbd541ecba5d6640077618e9dfec6f5e71132
change-id: 20261111-bug-mm-shmem-undo-frame-66828e197ad5

Best regards,
-- 
Qiliang Yuan <qiliang.yuan@linux.dev>