On 8 January 2016 at 13:00, Catalin Marinas [off-list ref] wrote:

On Wed, Dec 30, 2015 at 04:26:03PM +0100, Ard Biesheuvel wrote:

quoted

@@ -583,33 +555,42 @@ void __init early_fixmap_init(void)
      unsigned long addr = FIXADDR_START;

      pgd = pgd_offset_k(addr);
-     pgd_populate(&init_mm, pgd, bm_pud);
-     pud = pud_offset(pgd, addr);
-     pud_populate(&init_mm, pud, bm_pmd);
-     pmd = pmd_offset(pud, addr);
-     pmd_populate_kernel(&init_mm, pmd, bm_pte);
+#if CONFIG_PGTABLE_LEVELS > 3
+     if (pgd_none(*pgd)) {
+             static pud_t bm_pud[PTRS_PER_PUD] __pgdir;
+
+             pgd_populate(&init_mm, pgd, bm_pud);
+             memblock_reserve(__pa(bm_pud), sizeof(bm_pud));
+     }
+     pud = (pud_t *)__phys_to_kimg(pud_offset_phys(pgd, addr));
+#else
+     pud = (pud_t *)pgd;
+#endif
+#if CONFIG_PGTABLE_LEVELS > 2
+     if (pud_none(*pud)) {
+             static pmd_t bm_pmd[PTRS_PER_PMD] __pgdir;
+
+             pud_populate(&init_mm, pud, bm_pmd);
+             memblock_reserve(__pa(bm_pmd), sizeof(bm_pmd));
+     }
+     pmd = (pmd_t *)__phys_to_kimg(pmd_offset_phys(pud, addr));
+#else
+     pmd = (pmd_t *)pud;
+#endif
+     if (pmd_none(*pmd)) {
+             static pte_t bm_pte[PTRS_PER_PTE] __pgdir;
+
+             pmd_populate_kernel(&init_mm, pmd, bm_pte);
+             memblock_reserve(__pa(bm_pte), sizeof(bm_pte));
+     }
+     __fixmap_pte = (pte_t *)__phys_to_kimg(pmd_page_paddr(*pmd));

I haven't tried but could you not avoid the #if and just rely on the
pud_none() etc. definitions to be 0 and the compiler+linker optimising
the irrelevant code out?

I tried but it requires some tinkering so I gave up. I can have
another go based on the latest version (which will look a bit
different)