[PATCH v6 09/31] ext4: skip block allocation for holes in the data submission path

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

When ext4_map_blocks() is called from the data submission path and I/O
end extent conversion path (EXT4_GET_BLOCKS_IO_SUBMIT), it should not
allocate blocks if the lookup returns a hole.

The writeback path can legitimately encounter dirty ranges that map to
holes. For example, when a folio straddles i_size and the tail beyond
i_size is dirtied via a mmap write. Allocating blocks for such ranges is
wrong because there is no data to write back, the dirty bits should
simply be discarded without submitting I/O. This prepares for the
buffered iomap writeback conversion, mirrors the existing buffer_head
writeback path, where mpage_add_bh_to_extent() skip unmapped buffers and
ext4_bio_write_folio() clears their dirty bits.

In the ioend extent conversion path, holes are also not expected because
we should wait for folio writeback before punching hole. If one is
encountered, it likely indicates a failure in the concurrency
protection, so ext4_map_blocks() returns zero, and we keep warning and
bail out with -EINVAL to surface the failure rather than continuing
conversion on torn data. Atomic writes in
ext4_convert_unwritten_extents_atomic() already bail out similarly.

Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
---
 fs/ext4/inode.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7a5c74af8ff3..2881596bff8b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -825,6 +825,13 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 			map->m_flags |= EXT4_MAP_MAPPED;
 			goto out_handle;
 		}
+	} else if (retval == 0) {
+		/*
+		 * Do not allocate blocks for holes in the context of
+		 * data submission path.
+		 */
+		if (!map->m_flags && (flags & EXT4_GET_BLOCKS_IO_SUBMIT))
+			goto out_handle;
 	}
 
 	if (!handle) {
-- 
2.52.0