x86-64: fall back to regular page vmemmap on allocation failure

Memory hotplug can happen on a machine under load, memory shortness
and fragmentation, so huge page allocations for the vmemmap are not
guaranteed to succeed.

Try to fall back to regular pages before failing the hotplug event
completely.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Bernhard Schmidt <Bernhard.Schmidt@lrz.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/mm/init_64.c

@@ -1303,12 +1303,11 @@ static int __meminit vmemmap_populate_hugepages(unsigned long start,
 
 		pmd = pmd_offset(pud, addr);
 		if (pmd_none(*pmd)) {
-			pte_t entry;
 			void *p;
 
 			p = vmemmap_alloc_block_buf(PMD_SIZE, node);
-			if (!p)
-				return -ENOMEM;
+			if (p) {
+				pte_t entry;
 
 				entry = pfn_pte(__pa(p) >> PAGE_SHIFT,
 						PAGE_KERNEL_LARGE);
@@ -1326,8 +1325,15 @@ static int __meminit vmemmap_populate_hugepages(unsigned long start,
 
 				addr_end = addr + PMD_SIZE;
 				p_end = p + PMD_SIZE;
-		} else
+				continue;
+			}
+		} else if (pmd_large(*pmd)) {
 			vmemmap_verify((pte_t *)pmd, node, addr, next);
+			continue;
+		}
+		pr_warn_once("vmemmap: falling back to regular page backing\n");
+		if (vmemmap_populate_basepages(addr, next, node))
+			return -ENOMEM;
 	}
 	return 0;
 }