mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty_slowpath()

As commit 1836522 ("hwpoison, memcg: forcibly uncharge LRU pages"),
hwpoison will forcibly uncharg a LRU hwpoisoned page, the folio_memcg
could be NULl, then, mem_cgroup_track_foreign_dirty_slowpath() could
occurs a NULL pointer dereference, let's do not record the foreign
writebacks for folio memcg is null in mem_cgroup_track_foreign_dirty() to
fix it.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 97b2782 ("writeback, memcg: Implement foreign dirty flushing")
Signed-off-by: Kefeng Wang <[email protected]>
Reported-by: Ma Wupeng <[email protected]>
Tested-by: Miko Larsson <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Ma Wupeng <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kefeng Wang

include/linux/memcontrol.h

@@ -1666,10 +1666,13 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
 static inline void mem_cgroup_track_foreign_dirty(struct folio *folio,
 						  struct bdi_writeback *wb)
 {
+	struct mem_cgroup *memcg;
+
 	if (mem_cgroup_disabled())
 		return;
 
-	if (unlikely(&folio_memcg(folio)->css != wb->memcg_css))
+	memcg = folio_memcg(folio);
+	if (unlikely(memcg && &memcg->css != wb->memcg_css))
 		mem_cgroup_track_foreign_dirty_slowpath(folio, wb);
 }