Function abd_get_offset_impl

freebsd/contrib/openzfs/module/zfs/abd.c:522–565

* Allocate a new ABD to point to offset off of sabd. It shares the underlying * buffer data with sabd. Use abd_put() to free. sabd must not be freed while * any derived ABDs exist. */

 * any derived ABDs exist.
 */
static abd_t *
abd_get_offset_impl(abd_t *sabd, size_t off, size_t size)
{
	abd_t *abd = NULL;

	abd_verify(sabd);
	ASSERT3U(off, <=, sabd->abd_size);

	if (abd_is_linear(sabd)) {
		abd = abd_alloc_struct(0);

		/*
		 * Even if this buf is filesystem metadata, we only track that
		 * if we own the underlying data buffer, which is not true in
		 * this case. Therefore, we don't ever use ABD_FLAG_META here.
		 */
		abd->abd_flags = ABD_FLAG_LINEAR;

		ABD_LINEAR_BUF(abd) = (char *)ABD_LINEAR_BUF(sabd) + off;
	} else if (abd_is_gang(sabd)) {
		size_t left = size;
		abd = abd_alloc_gang_abd();
		abd->abd_flags &= ~ABD_FLAG_OWNER;
		for (abd_t *cabd = abd_gang_get_offset(sabd, &off);
		    cabd != NULL && left > 0;
		    cabd = list_next(&ABD_GANG(sabd).abd_gang_chain, cabd)) {
			int csize = MIN(left, cabd->abd_size - off);

			abd_t *nabd = abd_get_offset_impl(cabd, off, csize);
			abd_gang_add(abd, nabd, B_FALSE);
			left -= csize;
			off = 0;
		}
		ASSERT3U(left, ==, 0);
	} else {
		abd = abd_get_offset_scatter(sabd, off);
	}

	abd->abd_size = size;
	abd->abd_parent = sabd;
	zfs_refcount_create(&abd->abd_children);
	(void) zfs_refcount_add_many(&sabd->abd_children, abd->abd_size, abd);
	return (abd);
}

abd_t *
abd_get_offset(abd_t *sabd, size_t off)