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.

Reported-by: Ma Wupeng <mawupeng1@huawei.com>
Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
resend: correct function name
 include/linux/memcontrol.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index eb6e5b18e1ad..35478695cabf 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1688,10 +1688,13 @@ 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);
 }
 
-- 
2.35.3

On Sun, 2023-01-29 at 12:09 +0800, Kefeng Wang wrote:
> 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.
> 
> Reported-by: Ma Wupeng <mawupeng1@huawei.com>
> Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty
> flushing")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> resend: correct function name
>  include/linux/memcontrol.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index eb6e5b18e1ad..35478695cabf 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1688,10 +1688,13 @@ 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);
>  }
>  

Might want to Cc linux-stable.
Tested-by: Miko Larsson <mikoxyzzz@gmail.com>

--
~miko