[PATCH] make sure all PMDs are allocated under PAE mode

Patch from Martin Bligh and Dave Hansen

If a PAE machine has 1G of memory and you set PAGE_OFFSET to 2G, the
kernel will only instantiate a PMD to cover the 2G-3G region.  But
another PMD is needed for the 3G-4G region for the APIC and possibly an
extended vmalloc region.

So the patch changes the code to instantiate PMDs out to the end of
physical memory.

It's a no-op for PAGE_OFFSET=3G, and _could_ be part of the
CONFIG_PAGE_OFFSET patch.  But it seems a reasonable generalisation
anyway.

arch/i386/mm/init.c

@@ -134,8 +134,10 @@ static void __init kernel_physical_mapping_init(pgd_t *pgd_base)
 	pgd = pgd_base + pgd_ofs;
 	pfn = 0;
 
-	for (; pgd_ofs < PTRS_PER_PGD && pfn < max_low_pfn; pgd++, pgd_ofs++) {
+	for (; pgd_ofs < PTRS_PER_PGD; pgd++, pgd_ofs++) {
 		pmd = one_md_table_init(pgd);
+		if (pfn >= max_low_pfn)
+			continue;
 		for (pmd_ofs = 0; pmd_ofs < PTRS_PER_PMD && pfn < max_low_pfn; pmd++, pmd_ofs++) {
 			/* Map with big pages if possible, otherwise create normal page tables. */
 			if (cpu_has_pse) {