hugetlb, x86: register 1G page size if we can allocate them at runtime

After commit 944d9fec ("hugetlb: add support for gigantic page
allocation at runtime") we can allocate 1G pages at runtime if CMA is
enabled.

Let's register 1G pages into hugetlb even if the user hasn't requested
them explicitly at boot time with hugepagesz=1G.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/mm/hugetlbpage.c

@@ -178,4 +178,15 @@ static __init int setup_hugepagesz(char *opt)
 	return 1;
 }
 __setup("hugepagesz=", setup_hugepagesz);
+
+#ifdef CONFIG_CMA
+static __init int gigantic_pages_init(void)
+{
+	/* With CMA we can allocate gigantic pages at runtime */
+	if (cpu_has_gbpages && !size_to_hstate(1UL << PUD_SHIFT))
+		hugetlb_add_hstate(PUD_SHIFT - PAGE_SHIFT);
+	return 0;
+}
+arch_initcall(gigantic_pages_init);
+#endif
 #endif