Btrfs: [PATCH] extent_map: add writepage_end_io hook

XFS updates the ondisk inode size only after the data I/O has finished,
so it needs a hook when the writepage end_bio handler has finished.

Might not be worth applying as-is as the per-page callback is very
ineffcient.  What XFS really wants is a callback when writeout of a
whole extent has completed.  This delayed i_size updates scheme might
be worthwile for btrfs aswell, btw.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_map.c

@@ -1231,6 +1231,8 @@ static int end_bio_extent_writepage(struct bio *bio,
 			end_page_writeback(page);
 		else
 			check_page_writeback(tree, page);
+		if (tree->ops && tree->ops->writepage_end_io_hook)
+			tree->ops->writepage_end_io_hook(page, start, end);
 	} while (bvec >= bio->bi_io_vec);
 
 	bio_put(bio);

fs/btrfs/extent_map.h

@@ -11,6 +11,7 @@ struct extent_map_ops {
 	int (*writepage_io_hook)(struct page *page, u64 start, u64 end);
 	int (*readpage_io_hook)(struct page *page, u64 start, u64 end);
 	int (*readpage_end_io_hook)(struct page *page, u64 start, u64 end);
+	void (*writepage_end_io_hook)(struct page *page, u64 start, u64 end);
 };
 
 struct extent_map_tree {