[PATCH v2] mm/page_owner: Don't access uninitialized memmaps when reading /proc/pagetypeinfo

[David Hildenbrand <david@redhat.com>]

From: Qian Cai <cai@lca.pw>

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 not 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).

Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visible after d0dc12e86b319
Signed-off-by: Qian Cai <cai@lca.pw>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
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>
[ move check to outer loop, add comment, rephrase description ]
Signed-off-by: David Hildenbrand <david@redhat.com>
---

Cai asked me to follow up on:
	[PATCH] mm/page_owner: fix a crash after memory offline

---
 mm/page_owner.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index dee931184788..7d149211f6be 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -284,7 +284,8 @@ 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;
 		}
@@ -292,13 +293,13 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
 		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)
-- 
2.21.0