Uninitialized memmaps contain garbage and in the worst case trigger kernel
BUGs, especially with CONFIG_PAGE_POISONING.  They should not get touched.

For example, when not onlining a memory block that is spanned by a zone
and reading /proc/pagetypeinfo with CONFIG_DEBUG_VM_PGFLAGS and
CONFIG_PAGE_POISONING, we can trigger a kernel BUG:

:/# echo 1 > /sys/devices/system/memory/memory40/online
:/# echo 1 > /sys/devices/system/memory/memory42/online
:/# cat /proc/pagetypeinfo > test.file
  [   42.489856] page:fffff2c585200000 is uninitialized and poisoned
  [   42.489861] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
  [   42.492235] raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
  [   42.493501] page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
  [   42.494533] There is not page extension available.
  [   42.495358] ------------[ cut here ]------------
  [   42.496163] kernel BUG at include/linux/mm.h:1107!
  [   42.497069] invalid opcode: 0000 [#1] SMP NOPTI

Please note that this change does not affect ZONE_DEVICE, because
pagetypeinfo_showmixedcount_print() is called from
mm/vmstat.c:pagetypeinfo_showmixedcount() only for populated zones, and
ZONE_DEVICE is never populated (zone->present_pages always 0).

[david@redhat.com: move check to outer loop, add comment, rephrase description]
Link: http://lkml.kernel.org/r/20191011140638.8160-1-david@redhat.com
Fixes: f1dd2cd1 ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visible after d0dc12e8


Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Miles Chen <miles.chen@mediatek.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>	[4.13+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>