mm/kasan/kasan_init.c: use kasan_zero_pud for p4d table

There is missing optimization in zero_p4d_populate() that can save some
memory when mapping zero shadow.  Implement it like as others.

Link: http://lkml.kernel.org/r/1494829255-23946-1-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/kasan_init.c

@@ -118,6 +118,18 @@ static void __init zero_p4d_populate(pgd_t *pgd, unsigned long addr,
 
 	do {
 		next = p4d_addr_end(addr, end);
+		if (IS_ALIGNED(addr, P4D_SIZE) && end - addr >= P4D_SIZE) {
+			pud_t *pud;
+			pmd_t *pmd;
+
+			p4d_populate(&init_mm, p4d, lm_alias(kasan_zero_pud));
+			pud = pud_offset(p4d, addr);
+			pud_populate(&init_mm, pud, lm_alias(kasan_zero_pmd));
+			pmd = pmd_offset(pud, addr);
+			pmd_populate_kernel(&init_mm, pmd,
+						lm_alias(kasan_zero_pte));
+			continue;
+		}
 
 		if (p4d_none(*p4d)) {
 			p4d_populate(&init_mm, p4d,