x86/e820: Use much less memory for e820/e820_saved, save up to 120k

The maximum size of e820 map array for EFI systems is defined as
E820_X_MAX (E820MAX + 3 * MAX_NUMNODES).

In x86_64 defconfig, this ends up with E820_X_MAX = 320, e820 and e820_saved
are 6404 bytes each.

With larger configs, for example Fedora kernels, E820_X_MAX = 3200, e820
and e820_saved are 64004 bytes each. Most of this space is wasted.
Typical machines have some 20-30 e820 areas at most.

After previous patch, e820 and e820_saved are pointers to e280 maps.

Change them to initially point to maps which are __initdata.

At the very end of kernel init, just before __init[data] sections are freed
in free_initmem(), allocate smaller blocks, copy maps there,
and change pointers.

The late switch makes sure that all functions which can be used to change
e820 maps are no longer accessible (they are all __init functions).

Run-tested.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20160918182125.21000-1-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/e820.c

@@ -40,10 +40,10 @@
  * user can e.g. boot the original kernel with mem=1G while still booting the
  * next kernel with full memory.
  */
-static struct e820map initial_e820;
-static struct e820map initial_e820_saved;
-struct e820map *e820 = &initial_e820;
-struct e820map *e820_saved = &initial_e820_saved;
+static struct e820map initial_e820  __initdata;
+static struct e820map initial_e820_saved  __initdata;
+struct e820map *e820 __refdata = &initial_e820;
+struct e820map *e820_saved __refdata = &initial_e820_saved;
 
 /* For PCI or other memory-mapped resources */
 unsigned long pci_mem_start = 0xaeedbabe;

arch/x86/mm/init.c

@@ -699,9 +699,9 @@ void free_init_pages(char *what, unsigned long begin, unsigned long end)
 	}
 }
 
-void free_initmem(void)
+void __ref free_initmem(void)
 {
-	/* e820_reallocate_tables(); - disabled for now */
+	e820_reallocate_tables();
 
 	free_init_pages("unused kernel",
 			(unsigned long)(&__init_begin),