btrfs: add optimized version of eb to eb copy

Using copy_extent_buffer is suitable for copying betwenn buffers from an
arbitrary offset and deals with page boundaries. This is not necessary
when doing a full extent_buffer-to-extent_buffer copy. We can utilize
the copy_page helper as well.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -260,7 +260,7 @@ int btrfs_copy_root(struct btrfs_trans_handle *trans,
 	if (IS_ERR(cow))
 		return PTR_ERR(cow);
 
-	copy_extent_buffer(cow, buf, 0, 0, cow->len);
+	copy_extent_buffer_full(cow, buf);
 	btrfs_set_header_bytenr(cow, cow->start);
 	btrfs_set_header_generation(cow, trans->transid);
 	btrfs_set_header_backref_rev(cow, BTRFS_MIXED_BACKREF_REV);
@@ -1129,7 +1129,7 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans,
 
 	/* cow is set to blocking by btrfs_init_new_buffer */
 
-	copy_extent_buffer(cow, buf, 0, 0, cow->len);
+	copy_extent_buffer_full(cow, buf);
 	btrfs_set_header_bytenr(cow, cow->start);
 	btrfs_set_header_generation(cow, trans->transid);
 	btrfs_set_header_backref_rev(cow, BTRFS_MIXED_BACKREF_REV);

fs/btrfs/extent_io.c

@@ -5546,6 +5546,20 @@ void memzero_extent_buffer(struct extent_buffer *eb, unsigned long start,
 	}
 }
 
+void copy_extent_buffer_full(struct extent_buffer *dst,
+			     struct extent_buffer *src)
+{
+	int i;
+	unsigned num_pages;
+
+	ASSERT(dst->len == src->len);
+
+	num_pages = num_extent_pages(dst->start, dst->len);
+	for (i = 0; i < num_pages; i++)
+		copy_page(page_address(dst->pages[i]),
+				page_address(src->pages[i]));
+}
+
 void copy_extent_buffer(struct extent_buffer *dst, struct extent_buffer *src,
 			unsigned long dst_offset, unsigned long src_offset,
 			unsigned long len)

fs/btrfs/extent_io.h

@@ -410,6 +410,8 @@ void write_extent_buffer_chunk_tree_uuid(struct extent_buffer *eb,
 		const void *src);
 void write_extent_buffer(struct extent_buffer *eb, const void *src,
 			 unsigned long start, unsigned long len);
+void copy_extent_buffer_full(struct extent_buffer *dst,
+			     struct extent_buffer *src);
 void copy_extent_buffer(struct extent_buffer *dst, struct extent_buffer *src,
 			unsigned long dst_offset, unsigned long src_offset,
 			unsigned long len);