3 files changed, 26 insertions, 11 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 67d663ab658a..60d4f1b210d7 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -4653,9 +4653,12 @@ static int btrfs_cleanup_transaction(struct btrfs_fs_info *fs_info)
 }
 
 static const struct extent_io_ops btree_extent_io_ops = {
-        .readpage_end_io_hook = btree_readpage_end_io_hook,
-        .readpage_io_failed_hook = btree_io_failed_hook,
+        /* mandatory callbacks */
         .submit_bio_hook = btree_submit_bio_hook,
+        .readpage_end_io_hook = btree_readpage_end_io_hook,
         /* note we're sharing with inode.c for the merge bio hook */
         .merge_bio_hook = btrfs_merge_bio_hook,
+
+        /* optional callbacks */
+        .readpage_io_failed_hook = btree_io_failed_hook,
 };
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,
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b904c74f31fd..3b327c8cfb16 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -10547,10 +10547,13 @@ static const struct file_operations btrfs_dir_file_operations = {
 };
 
 static const struct extent_io_ops btrfs_extent_io_ops = {
-        .fill_delalloc = run_delalloc_range,
+        /* mandatory callbacks */
         .submit_bio_hook = btrfs_submit_bio_hook,
-        .merge_bio_hook = btrfs_merge_bio_hook,
         .readpage_end_io_hook = btrfs_readpage_end_io_hook,
+        .merge_bio_hook = btrfs_merge_bio_hook,
+
+        /* optional callbacks */
+        .fill_delalloc = run_delalloc_range,
         .writepage_end_io_hook = btrfs_writepage_end_io_hook,
         .writepage_start_hook = btrfs_writepage_start_hook,
         .set_bit_hook = btrfs_set_bit_hook,