mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty_slowpath()

mainline inclusion
from mainline-v6.2-rc7
commit ac86f547
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BYND

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ac86f547ca1002aec2ef66b9e64d03f45bbbfbb9

--------------------------------

As commit 18365225 ("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: 97b27821 ("writeback, memcg: Implement foreign dirty flushing")
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reported-by: NMa Wupeng <mawupeng1@huawei.com>
Tested-by: NMiko Larsson <mikoxyzzz@gmail.com>
Acked-by: NMichal 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: NAndrew Morton <akpm@linux-foundation.org>
Conflicts:
	include/linux/memcontrol.h
Signed-off-by: NLu Jialin <lujialin4@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: NWang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: NJialin Zhang <zhangjialin11@huawei.com>

include/linux/memcontrol.h

@@ -1873,10 +1873,13 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
 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);
 }