Btrfs: Checksum tree blocks in the background

Tree blocks were using async bio submission, but the sum was still
being done directly during writepage.  This moves the checksumming
into the worker thread.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -315,14 +315,6 @@ int csum_dirty_buffer(struct btrfs_root *root, struct page *page)
 	return 0;
 }
 
-static int btree_writepage_io_hook(struct page *page, u64 start, u64 end)
-{
-	struct btrfs_root *root = BTRFS_I(page->mapping->host)->root;
-
-	csum_dirty_buffer(root, page);
-	return 0;
-}
-
 int btree_readpage_end_io_hook(struct page *page, u64 start, u64 end,
 			       struct extent_state *state)
 {
@@ -501,6 +493,22 @@ int btrfs_wq_submit_bio(struct btrfs_fs_info *fs_info, struct inode *inode,
 	return 0;
 }
 
+static int btree_csum_one_bio(struct bio *bio)
+{
+	struct bio_vec *bvec = bio->bi_io_vec;
+	int bio_index = 0;
+	struct btrfs_root *root;
+
+	WARN_ON(bio->bi_vcnt <= 0);
+	while(bio_index < bio->bi_vcnt) {
+		root = BTRFS_I(bvec->bv_page->mapping->host)->root;
+		csum_dirty_buffer(root, bvec->bv_page);
+		bio_index++;
+		bvec++;
+	}
+	return 0;
+}
+
 static int __btree_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
 				 int mirror_num)
 {
@@ -515,6 +523,7 @@ static int __btree_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
 	 * submission context.  Just jump into btrfs_map_bio
 	 */
 	if (rw & (1 << BIO_RW)) {
+		btree_csum_one_bio(bio);
 		return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio,
 				     mirror_num, 1);
 	}
@@ -2040,7 +2049,6 @@ int btree_lock_page_hook(struct page *page)
 
 static struct extent_io_ops btree_extent_io_ops = {
 	.write_cache_pages_lock_hook = btree_lock_page_hook,
-	.writepage_io_hook = btree_writepage_io_hook,
 	.readpage_end_io_hook = btree_readpage_end_io_hook,
 	.submit_bio_hook = btree_submit_bio_hook,
 	/* note we're sharing with inode.c for the merge bio hook */