PHSIZE
if (pp->pr_size < MIN(pagesize / 16, PHSIZE * 8)) {
if (pagesize / pp->pr_size == (pagesize - PHSIZE) / pp->pr_size) {
bmapsize = roundup(PHSIZE, pp->pr_align) -
itemspace = palloc->pa_pagesz - roundup(PHSIZE, align);
pp->pr_itemoffset = roundup(PHSIZE, align);