btrfs: document existence of extent_io ops callbacks

Some of the callbacks defined in btree_extent_io_ops and
btrfs_extent_io_ops do always exist so we don't need to check the
existence before each call. This patch just reorders the definition and
documents which are mandatory/optional.

Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 78bb4d76127b..1f8478dc9f8e 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -92,18 +92,27 @@ typedef	int (extent_submit_bio_hook_t)(struct inode *inode, struct bio *bio,
 				       int mirror_num, unsigned long bio_flags,
 				       u64 bio_offset);
 struct extent_io_ops {
-	int (*fill_delalloc)(struct inode *inode, struct page *locked_page,
-			     u64 start, u64 end, int *page_started,
-			     unsigned long *nr_written);
-	int (*writepage_start_hook)(struct page *page, u64 start, u64 end);
+	/*
+	 * The following callbacks must be allways defined, the function
+	 * pointer will be called unconditionally.
+	 */
 	extent_submit_bio_hook_t *submit_bio_hook;
+	int (*readpage_end_io_hook)(struct btrfs_io_bio *io_bio, u64 phy_offset,
+				    struct page *page, u64 start, u64 end,
+				    int mirror);
 	int (*merge_bio_hook)(struct page *page, unsigned long offset,
 			      size_t size, struct bio *bio,
 			      unsigned long bio_flags);
+
+	/*
+	 * Optional hooks, called if the pointer is not NULL
+	 */
+	int (*fill_delalloc)(struct inode *inode, struct page *locked_page,
+			     u64 start, u64 end, int *page_started,
+			     unsigned long *nr_written);
 	int (*readpage_io_failed_hook)(struct page *page, int failed_mirror);
-	int (*readpage_end_io_hook)(struct btrfs_io_bio *io_bio, u64 phy_offset,
-				    struct page *page, u64 start, u64 end,
-				    int mirror);
+
+	int (*writepage_start_hook)(struct page *page, u64 start, u64 end);
 	void (*writepage_end_io_hook)(struct page *page, u64 start, u64 end,
 				      struct extent_state *state, int uptodate);
 	void (*set_bit_hook)(struct inode *inode, struct extent_state *state,