[PATCH] mm/shmem: remove truncated folios from the page cache in batches

[Qiliang Yuan <odys.yuan@gmail.com>]

truncate_inode_pages_range() takes the folios it finds out of the page
cache with delete_from_page_cache_batch(), which takes the inode's
i_lock and the i_pages lock once for each batch of up to
FOLIO_BATCH_SIZE folios.

shmem_undo_range() calls truncate_inode_folio() on each folio instead,
so filemap_remove_folio() takes and drops both locks once per folio.
Truncating, punching or freeing a tmpfs file or memfd made of small
folios, such as a large /dev/shm segment, pays for that on every page.

Clean each folio up as truncate_inode_folio() does, collect the ones to
remove and take them out with delete_from_page_cache_batch(), which
needs truncate_cleanup_folio() outside mm/truncate.c. Swap entries, and
the folios kept when undoing a failed fallocate(), stay out of the
batch. Dropping an 8 GiB file of 4K folios, median of five runs,
averaged over two boots of each kernel, in a 32 vCPU VM:

                        before    after
  ftruncate() to 0     1163 ms   913 ms
  hole punch           1155 ms   914 ms
  close() of a memfd   1161 ms   914 ms
  unlink()             1162 ms   916 ms

Files of PMD-sized folios take 20-25 ms with or without this change.

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
To see it, fill and remove a large /dev/shm file:

  fallocate -l 8G /dev/shm/f && time rm /dev/shm/f
---
 mm/internal.h |  1 +
 mm/shmem.c    | 16 +++++++++++++---
 mm/truncate.c |  2 +-
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 566cbd2dce485..abe64e49a4170 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -628,6 +628,7 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
 		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
 unsigned find_get_entries(struct address_space *mapping, pgoff_t *start,
 		pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
+void truncate_cleanup_folio(struct folio *folio);
 int truncate_inode_folio(struct address_space *mapping, struct folio *folio);
 bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart,
 		loff_t lend, pgoff_t *pstart, pgoff_t *pend);
diff --git a/mm/shmem.c b/mm/shmem.c
index af21a89abb0f6..16b6e38503bc3 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1352,6 +1352,9 @@ 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];
 
@@ -1363,9 +1366,16 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				continue;
 			}
 
-			if (!unfalloc || !folio_test_uptodate(folio))
-				truncate_inode_folio(mapping, folio);
-			folio_unlock(folio);
+			if (!unfalloc || !folio_test_uptodate(folio)) {
+				truncate_cleanup_folio(folio);
+				folio_batch_add(&remove, folio);
+			}
+		}
+		delete_from_page_cache_batch(mapping, &remove);
+		for (i = 0; i < folio_batch_count(&fbatch); i++) {
+			folio = fbatch.folios[i];
+			if (!xa_is_value(folio))
+				folio_unlock(folio);
 		}
 		folio_batch_remove_exceptionals(&fbatch);
 		folio_batch_release(&fbatch);
diff --git a/mm/truncate.c b/mm/truncate.c
index 733d2713a875a..c6bf4aa0b91e8 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -151,7 +151,7 @@ EXPORT_SYMBOL_GPL(folio_invalidate);
  * its lock, b) when a concurrent invalidate_mapping_pages got there first and
  * c) when tmpfs swizzles a page between a tmpfs inode and swapper_space.
  */
-static void truncate_cleanup_folio(struct folio *folio)
+void truncate_cleanup_folio(struct folio *folio)
 {
 	if (folio_mapped(folio))
 		unmap_mapping_folio(folio);

---
base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
change-id: 20261029-bug-mm-shmem-undo-batch-de001d9c6f1e

Best regards,
-- 
Qiliang Yuan <odys.yuan@gmail.com>