SG_PAGE_LINK_LAST
SG_PAGE_LINK_CHAIN) & ~SG_PAGE_LINK_LAST;
sg->page_link |= SG_PAGE_LINK_LAST;
#define SG_END SG_PAGE_LINK_LAST
#define sg_is_last(sg) ((sg)->page_link & SG_PAGE_LINK_LAST)