[PATCH -next v5 10/32] ext4: skip block allocation for holes in the data submission path

From: Zhang Yi <[email protected]>

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 mirrors the existing
buffer_head writeback path, where mpage_add_bh_to_extent() skips
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. In this case, to avoid losing data beyond the hole, do not
stop conversion, continue on the remaining ranges. This prepares for the
buffered iomap writeback conversion.

Signed-off-by: Zhang Yi <[email protected]>
---
 fs/ext4/extents.c | 8 ++++++--
 fs/ext4/inode.c   | 7 +++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 76038b6c3655..0d62d9312284 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5167,11 +5167,15 @@ int ext4_convert_unwritten_extents(handle_t *handle, struct inode *inode,
 				      EXT4_GET_BLOCKS_IO_CONVERT_EXT |
 				      EXT4_EX_NOCACHE);
 		if (ret <= 0) {
+			/*
+			 * If the ret is zero, an unexpected hole may cause
+			 * conversion to fail.  To avoid data loss during I/O
+			 * end conversion, skip the hole and continue
+			 * converting subsequent blocks.
+			 */
 			ext4_warning(inode->i_sb,
 				     "inode #%llu: block %u: len %u: ext4_map_blocks returned %d",
 				     inode->i_ino, map.m_lblk, map.m_len, ret);
-			if (unlikely(ret == 0))
-				ret = -EINVAL;
 		} else {
 			conv_blocks += map.m_len;
 		}
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5dcc3f7b2ffd..d8c3e5e13b8a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -823,6 +823,13 @@ 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