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    SubjectRe: [PATCH v2] memcg: make it work on sparse non-0-node systems
    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@suse.cz>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > Cc: Michal Hocko <mhocko@kernel.org>
    > Suggested-by: Vladimir Davydov <vdavydov.dev@gmail.com>
    > Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
    > Cc: <cgroups@vger.kernel.org>
    > Cc: <linux-mm@kvack.org>
    > Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>

    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@suse.com>

    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

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