PAGES_PER_SGE_SHIFT
#define PAGES_PER_SGE (1 << PAGES_PER_SGE_SHIFT)
pool->page = alloc_pages(gfp_mask, PAGES_PER_SGE_SHIFT);
(~(PAGES_PER_SGE-1))) >> PAGES_PER_SGE_SHIFT;
(~(PAGES_PER_SGE-1))) >> PAGES_PER_SGE_SHIFT;