On Fri 17-05-19 13:42:04, Jiri Slaby wrote:
> We have a single node system with node 0 disabled:
> Scanning NUMA topology in Northbridge 24
> Number of physical nodes 2
> Skipping disabled node 0
> Node 1 MemBase 0000000000000000 Limit 00000000fbff0000
> NODE_DATA(1) allocated [mem 0xfbfda000-0xfbfeffff]
>
> This causes crashes in memcg when system boots:
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> #PF error: [normal kernel read fault]
> ...
> RIP: 0010:list_lru_add+0x94/0x170
> ...
> Call Trace:
> d_lru_add+0x44/0x50
> dput.part.34+0xfc/0x110
> __fput+0x108/0x230
> task_work_run+0x9f/0xc0
> exit_to_usermode_loop+0xf5/0x100
>
> It is reproducible as far as 4.12. I did not try older kernels. You have
> to have a new enough systemd, e.g. 241 (the reason is unknown -- was not
> investigated). Cannot be reproduced with systemd 234.
>
> The system crashes because the size of lru array is never updated in
> memcg_update_all_list_lrus and the reads are past the zero-sized array,
> causing dereferences of random memory.
>
> The root cause are list_lru_memcg_aware checks in the list_lru code.
> The test in list_lru_memcg_aware is broken: it assumes node 0 is always
> present, but it is not true on some systems as can be seen above.
>
> So fix this by avoiding checks on node 0. Remember the memcg-awareness
> by a bool flag in struct list_lru.
>
> [v2] use the idea proposed by Vladimir -- the bool flag.
>
> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Suggested-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Acked-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: <cgroups@xxxxxxxxxxxxxxx>
> Cc: <linux-mm@xxxxxxxxx>
> Cc: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
Fixes: 60d3fd32a7a9 ("list_lru: introduce per-memcg lists")
unless I have missed something
Cc: stable sounds like a good idea to me as well, although nobody has
noticed this yet but Node0 machines are quite rare.
I haven't checked all users of list_lru but the structure size increase
shouldn't be a big problem. There tend to be only limited number of
those and the number shouldn't be huge.
So this looks good to me.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks a lot Jiri!
> ---
> include/linux/list_lru.h | 1 +
> mm/list_lru.c | 8 +++-----
> 2 files changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> index aa5efd9351eb..d5ceb2839a2d 100644
> --- a/include/linux/list_lru.h
> +++ b/include/linux/list_lru.h
> @@ -54,6 +54,7 @@ struct list_lru {
> #ifdef CONFIG_MEMCG_KMEM
> struct list_head list;
> int shrinker_id;
> + bool memcg_aware;
> #endif
> };
>
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 0730bf8ff39f..d3b538146efd 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -37,11 +37,7 @@ static int lru_shrinker_id(struct list_lru *lru)
>
> static inline bool list_lru_memcg_aware(struct list_lru *lru)
> {
> - /*
> - * This needs node 0 to be always present, even
> - * in the systems supporting sparse numa ids.
> - */
> - return !!lru->node[0].memcg_lrus;
> + return lru->memcg_aware;
> }
>
> static inline struct list_lru_one *
> @@ -451,6 +447,8 @@ static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
> {
> int i;
>
> + lru->memcg_aware = memcg_aware;
> +
> if (!memcg_aware)
> return 0;
>
> --
> 2.21.0
--
Michal Hocko
SUSE Labs
