[PATCH v3 2/3] tmpfs: zero post-eof folio ranges on swapout

[Brian Foster <bfoster@redhat.com>]

Zero post-eof folios at swap out time to help preserve POSIX
requirements and facilitate efficient post-eof zeroing during
extending operations. This uses an analogous approach to pagecache
writeback to ensure post-eof ranges are zeroed on-disk.

This facilitates file extension zeroing by allowing it to skip
swapped out folios that might have been preallocated beyond the
original i_size.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 mm/shmem.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 651602460770..97ca2b3dd1b9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1577,6 +1577,8 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 	struct inode *inode = mapping->host;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
+	loff_t i_size = i_size_read(inode);
+	pgoff_t end_index = DIV_ROUND_UP(i_size, PAGE_SIZE);
 	pgoff_t index;
 	int nr_pages;
 	bool split = false;
@@ -1596,9 +1598,9 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 	 * (unless fallocate has been used to preallocate beyond EOF).
 	 */
 	if (folio_test_large(folio)) {
-		index = shmem_fallocend(inode,
-			DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE));
-		if ((index > folio->index && index < folio_next_index(folio)) ||
+		pgoff_t fallocend = shmem_fallocend(inode, end_index);
+		if ((fallocend > folio->index &&
+		     fallocend < folio_next_index(folio)) ||
 		    !IS_ENABLED(CONFIG_THP_SWAP))
 			split = true;
 	}
@@ -1642,8 +1644,20 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 			goto redirty;
 	}
 
-	if (!folio_test_uptodate(folio)) {
-		folio_zero_range(folio, 0, folio_size(folio));
+	/*
+	 * Ranges beyond EOF must be zeroed at writeout time. This mirrors
+	 * traditional writeback behavior and facilitates zeroing on file size
+	 * changes without having to swap back in.
+	 */
+	if (!folio_test_uptodate(folio) ||
+	    folio_next_index(folio) >= end_index) {
+		size_t from = offset_in_folio(folio, i_size);
+
+		if (!folio_test_uptodate(folio) || index >= end_index)
+			folio_zero_segment(folio, 0, folio_size(folio));
+		else if (from)
+			folio_zero_segment(folio, from, folio_size(folio));
+
 		flush_dcache_folio(folio);
 		folio_mark_uptodate(folio);
 	}
-- 
2.51.1