mm/page_owner: don't access uninitialized memmaps when reading /proc/…

…pagetypeinfo

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
   page:fffff2c585200000 is uninitialized and poisoned
   raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
   raw: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
   page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
   There is not page extension available.
   ------------[ cut here ]------------
   kernel BUG at include/linux/mm.h:1107!
   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).

[[email protected]: move check to outer loop, add comment, rephrase description]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: f1dd2cd ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visible after d0dc12e
Signed-off-by: Qian Cai <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: Miles Chen <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Qian Cai <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: <[email protected]>	[4.13+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_owner.c

@@ -271,21 +271,22 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
 	 * not matter as the mixed block count will still be correct
 	 */
 	for (; pfn < end_pfn; ) {
-		if (!pfn_valid(pfn)) {
+		page = pfn_to_online_page(pfn);
+		if (!page) {
 			pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES);
 			continue;
 		}
 
 		block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
 		block_end_pfn = min(block_end_pfn, end_pfn);
 
-		page = pfn_to_page(pfn);
 		pageblock_mt = get_pageblock_migratetype(page);
 
 		for (; pfn < block_end_pfn; pfn++) {
 			if (!pfn_valid_within(pfn))
 				continue;
 
+			/* The pageblock is online, no need to recheck. */
 			page = pfn_to_page(pfn);
 
 			if (page_zone(page) != zone)