From: Kefeng Wang <wangkefeng.wang@huawei.com>
Date: Sun, 29 Jan 2023 12:09:45 +0800
Subject: mm: memcg: fix NULL pointer in
 mem_cgroup_track_foreign_dirty_slowpath()
Git-commit: ac86f547ca1002aec2ef66b9e64d03f45bbbfbb9
Patch-mainline: v6.2-rc7
References: bsc#1209262

As commit 18365225f044 ("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/20230129040945.180629-1-wangkefeng.wang@huawei.com
Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reported-by: Ma Wupeng <mawupeng1@huawei.com>
Tested-by: Miko Larsson <mikoxyzzz@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Ma Wupeng <mawupeng1@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
[mkoutny: Adjust folios to pages]
---
 include/linux/memcontrol.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1569,10 +1569,13 @@ void mem_cgroup_track_foreign_dirty_slow
 static inline void mem_cgroup_track_foreign_dirty(struct page *page,
 						  struct bdi_writeback *wb)
 {
+	struct mem_cgroup *memcg;
+
 	if (mem_cgroup_disabled())
 		return;
 
-	if (unlikely(&page_memcg(page)->css != wb->memcg_css))
+	memcg = page_memcg(page);
+	if (unlikely(memcg && &memcg->css != wb->memcg_css))
 		mem_cgroup_track_foreign_dirty_slowpath(page, wb);
 }