fixmap에 관하여.

 74 static phys_addr_t __init early_pgtable_alloc(void)
 75 {
 76     phys_addr_t phys;
 77     void *ptr;
 78 
 79     phys = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
 80     BUG_ON(!phys);
 81 
 82     /*
 83      * The FIX_{PGD,PUD,PMD} slots may be in active use, but the FIX_PTE
 84      * slot will be free, so we can (ab)use the FIX_PTE slot to initialise
 85      * any level of table.
 86      */
 87     ptr = pte_set_fixmap(phys);
 88 
 89     memset(ptr, 0, PAGE_SIZE);
 90 
 91     /*
 92      * Implicit barriers also ensure the zeroed page is visible to the page
 93      * table walker
 94      */
 95     pte_clear_fixmap();
 96 
 97     return phys;
 98 }