BUG_ON(PageReserved(page));
void page_remove_rmap(struct page *page){	BUG_ON(PageReserved(page));	if (atomic_add_negative(-1, &page->_mapcount)) {		BUG_ON(page_mapcount(page) < 0);		/*		 * It would be tidy to reset the PageAnon mapping here,		 * but that might overwrite a racing page_add_anon_rmap		 * which increments mapcount after us but sets mapping		 * before us: so leave the reset to free_hot_cold_page,		 * and remember that it's only reliable while mapped.		 * Leaving it set also helps swapoff to reinstate ptes		 * faster for those pages still in swapcache.		 */		if (page_test_and_clear_dirty(page))			set_page_dirty(page);		dec_page_state(nr_mapped);	}}