block: introduce bio_copy_data_partial

Turns out we can use bio_copy_data in raid1's write behind,
and we can make alloc_behind_pages() more clean/efficient,
but we need to partial version of bio_copy_data().
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>

block/bio.c

@@ -1025,19 +1025,8 @@ int bio_alloc_pages(struct bio *bio, gfp_t gfp_mask)
 }
 EXPORT_SYMBOL(bio_alloc_pages);
 
-/**
- * bio_copy_data - copy contents of data buffers from one chain of bios to
- * another
- * @src: source bio list
- * @dst: destination bio list
- *
- * If @src and @dst are single bios, bi_next must be NULL - otherwise, treats
- * @src and @dst as linked lists of bios.
- *
- * Stops when it reaches the end of either @src or @dst - that is, copies
- * min(src->bi_size, dst->bi_size) bytes (or the equivalent for lists of bios).
- */
-void bio_copy_data(struct bio *dst, struct bio *src)
+static void __bio_copy_data(struct bio *dst, struct bio *src,
+			    int offset, int size)
 {
 	struct bvec_iter src_iter, dst_iter;
 	struct bio_vec src_bv, dst_bv;
@@ -1047,6 +1036,12 @@ void bio_copy_data(struct bio *dst, struct bio *src)
 	src_iter = src->bi_iter;
 	dst_iter = dst->bi_iter;
 
+	/* for supporting partial copy */
+	if (offset || size != src->bi_iter.bi_size) {
+		bio_advance_iter(src, &src_iter, offset);
+		src_iter.bi_size = size;
+	}
+
 	while (1) {
 		if (!src_iter.bi_size) {
 			src = src->bi_next;
@@ -1083,8 +1078,47 @@ void bio_copy_data(struct bio *dst, struct bio *src)
 		bio_advance_iter(dst, &dst_iter, bytes);
 	}
 }
+
+/**
+ * bio_copy_data - copy contents of data buffers from one chain of bios to
+ * another
+ * @src: source bio list
+ * @dst: destination bio list
+ *
+ * If @src and @dst are single bios, bi_next must be NULL - otherwise, treats
+ * @src and @dst as linked lists of bios.
+ *
+ * Stops when it reaches the end of either @src or @dst - that is, copies
+ * min(src->bi_size, dst->bi_size) bytes (or the equivalent for lists of bios).
+ */
+void bio_copy_data(struct bio *dst, struct bio *src)
+{
+	__bio_copy_data(dst, src, 0, src->bi_iter.bi_size);
+}
 EXPORT_SYMBOL(bio_copy_data);
 
+/**
+ * bio_copy_data_partial - copy partial contents of data buffers from one
+ * chain of bios to another
+ * @dst: destination bio list
+ * @src: source bio list
+ * @offset: starting copy from the offset
+ * @size: how many bytes to copy
+ *
+ * If @src and @dst are single bios, bi_next must be NULL - otherwise, treats
+ * @src and @dst as linked lists of bios.
+ *
+ * Stops when it reaches the end of either @src or @dst - that is, copies
+ * min(src->bi_size, dst->bi_size) bytes (or the equivalent for lists of bios).
+ */
+void bio_copy_data_partial(struct bio *dst, struct bio *src,
+			   int offset, int size)
+{
+	__bio_copy_data(dst, src, offset, size);
+
+}
+EXPORT_SYMBOL(bio_copy_data_partial);
+
 struct bio_map_data {
 	int is_our_pages;
 	struct iov_iter iter;

include/linux/bio.h

@@ -468,6 +468,8 @@ static inline void bio_flush_dcache_pages(struct bio *bi)
 #endif
 
 extern void bio_copy_data(struct bio *dst, struct bio *src);
+extern void bio_copy_data_partial(struct bio *dst, struct bio *src,
+				  int offset, int size);
 extern int bio_alloc_pages(struct bio *bio, gfp_t gfp);
 extern void bio_free_pages(struct bio *bio);