x86/mm: use max memory block size on bare metal

Some of our servers spend significant time at kernel boot initializing
memory block sysfs directories and then creating symlinks between them and
the corresponding nodes.  The slowness happens because the machines get
stuck with the smallest supported memory block size on x86 (128M), which
results in 16,288 directories to cover the 2T of installed RAM.  The
search for each memory block is noticeable even with commit 4fb6eab
("drivers/base/memory.c: cache memory blocks in xarray to accelerate
lookup").

Commit 078eb6a ("x86/mm/memory_hotplug: determine block size based on
the end of boot memory") chooses the block size based on alignment with
memory end.  That addresses hotplug failures in qemu guests, but for bare
metal systems whose memory end isn't aligned to even the smallest size, it
leaves them at 128M.

Make kernels that aren't running on a hypervisor use the largest supported
size (2G) to minimize overhead on big machines.  Kernel boot goes 7%
faster on the aforementioned servers, shaving off half a second.

[[email protected]: v3]
  Link: http://lkml.kernel.org/r/[email protected]

Signed-off-by: Daniel Jordan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Sistare <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

arch/x86/mm/init_64.c

@@ -1452,6 +1452,15 @@ static unsigned long probe_memory_block_size(void)
 		goto done;
 	}
 
+	/*
+	 * Use max block size to minimize overhead on bare metal, where
+	 * alignment for memory hotplug isn't a concern.
+	 */
+	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
+		bz = MAX_BLOCK_SIZE;
+		goto done;
+	}
+
 	/* Find the largest allowed block size that aligns to memory end */
 	for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
 		if (IS_ALIGNED(boot_mem_end, bz))