More explanation about how "mm: accurately calculate zone->managed_pages for highmem zones" fixes the bug:
https://docs.google.com/document/d/1SGOHptN09gMD6P0bJwzSW0VtPgHCnjCAGH0G8XH0pJU/edit?usp=sharing

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/719e863357ccbb233d91b17853a5a746268c3612

commit 719e863357ccbb233d91b17853a5a746268c3612
Author: Jiang Liu <liuj97@gmail.com>
Date: Wed Oct 03 13:51:02 2018

UPSTREAM: mm: accurately calculate zone->managed_pages for highmem zones

Commit "mm: introduce new field 'managed_pages' to struct zone" assumes
that all highmem pages will be freed into the buddy system by function
mem_init().  But that's not always true, some architectures may reserve
some highmem pages during boot.  For example PPC may allocate highmem
pages for giagant HugeTLB pages, and several architectures have code to
check PageReserved flag to exclude highmem pages allocated during boot
when freeing highmem pages into the buddy system.

So treat highmem pages in the same way as normal pages, that is to:
1) reset zone->managed_pages to zero in mem_init().
2) recalculate managed_pages when freeing pages into the buddy system.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: <sworddragon2@aol.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jianguo Wu <wujianguo@huawei.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 7b4b2a0d6c8500350784beb83a6a55e60ea3bea3)
Signed-off-by: Kuo-Hsin Yang <vovoy@chromium.org>

BUG= chromium:891178 
TEST=boot and check /proc/zoneinfo
Change-Id: I4ba7e6d033514b5f854835490e9c44ed47c4adda
Reviewed-on: https://chromium-review.googlesource.com/1256642
Commit-Ready: Vovo Yang <vovoy@chromium.org>
Tested-by: Vovo Yang <vovoy@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>

[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/mm/nobootmem.c
[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/arch/metag/mm/init.c
[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/mm/bootmem.c
[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/arch/x86/mm/highmem_32.c
[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/mm/page_alloc.c
[modify] https://crrev.com/719e863357ccbb233d91b17853a5a746268c3612/include/linux/bootmem.h