PAGE_FRAG_CACHE_ORDER_MASK
#define PAGE_FRAG_CACHE_PFMEMALLOC_BIT (PAGE_FRAG_CACHE_ORDER_MASK + 1)
BUILD_BUG_ON(PAGE_FRAG_CACHE_MAX_ORDER > PAGE_FRAG_CACHE_ORDER_MASK);
(order & PAGE_FRAG_CACHE_ORDER_MASK) |
return encoded_page & PAGE_FRAG_CACHE_ORDER_MASK;