[PATCH v3 3/8] mm/mmzone: read each zoneref slot once when filtering by nodemask

["Gregory Price (Meta)" <gourry@gourry.net>]

__next_zones_zonelist() tests a slot's zone for NULL, then reads the
slot again to look up the zone's node. Offlining a zone's last block
rebuilds the zonelists in place, so the second read can return the
terminator:

  CPU 0                             CPU 1
  __next_zones_zonelist(nodemask)
    zonelist_zone(slot 1) = NORMAL
                                    offline_pages()
                                      build_all_zonelists()
                                        [MOVABLE, NORMAL, NULL]
                                        -> [NORMAL, NULL]
    zref_in_nodemask(slot 1)
      zone_to_nid(NULL)

Current GCC and clang merge the two reads, but nothing requires it. With
mm/mmzone.o built with -fno-inline -fno-ipa-sra, holding a nodemask
allocation between the reads while the node's last movable block goes
offline oopses every time:

  BUG: kernel NULL pointer dereference, address: 0000000000000060
  RIP: 0010:zone_to_nid+0x0/0x10
  Call Trace:
   zref_in_nodemask+0x9/0x20
   __next_zones_zonelist+0x23/0x70
   prepare_alloc_pages.constprop.0+0x16c/0x190
   __alloc_frozen_pages_noprof+0xaf/0x330

Read the slot once and test that zone. A NULL zone still ends the walk.

Fixes: 11cd8638c37f ("mm, page_alloc: remove stop_machine from build_all_zonelists")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/mmzone.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/mmzone.c b/mm/mmzone.c
index 9cc9ef588580d..009f1dd75dde8 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -43,13 +43,14 @@ struct zone *next_zone(struct zone *zone)
 	return zone;
 }
 
-static inline int zref_in_nodemask(struct zoneref *zref,
-				   const nodemask_t *nodes)
+/* A NULL zone terminates the zonelist, so it ends the search. */
+static inline bool zone_in_nodemask(const struct zone *zone,
+				    const nodemask_t *nodes)
 {
 #ifdef CONFIG_NUMA
-	return node_isset(zonelist_node_idx(zref), *nodes);
+	return !zone || node_isset(zone_to_nid(zone), *nodes);
 #else
-	return 1;
+	return true;
 #endif /* CONFIG_NUMA */
 }
 
@@ -67,7 +68,7 @@ struct zoneref *__next_zones_zonelist(struct zoneref *z,
 			z++;
 	else
 		while (zonelist_zone_idx(z) > highest_zoneidx ||
-				(zonelist_zone(z) && !zref_in_nodemask(z, nodes)))
+		       !zone_in_nodemask(READ_ONCE(z->zone), nodes))
 			z++;
 
 	return z;
-- 
2.53.0-Meta