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    SubjectRe: [PATCH] memcg: make it work on sparse non-0-node systems
    On Thu, May 9, 2019 at 5:25 AM Vladimir Davydov <vdavydov.dev@gmail.com> wrote:
    >
    > On Mon, Apr 29, 2019 at 12:59:39PM +0200, 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 checking the first online node instead of node 0.
    > >
    > > Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    > > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > > Cc: Michal Hocko <mhocko@kernel.org>
    > > Cc: 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>
    > > ---
    > > mm/list_lru.c | 6 +-----
    > > 1 file changed, 1 insertion(+), 5 deletions(-)
    > >
    > > diff --git a/mm/list_lru.c b/mm/list_lru.c
    > > index 0730bf8ff39f..7689910f1a91 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->node[first_online_node].memcg_lrus;
    > > }
    > >
    > > static inline struct list_lru_one *
    >
    > Yep, I didn't expect node 0 could ever be unavailable, my bad.
    > The patch looks fine to me:
    >
    > Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
    >
    > However, I tend to agree with Michal that (ab)using node[0].memcg_lrus
    > to check if a list_lru is memcg aware looks confusing. I guess we could
    > simply add a bool flag to list_lru instead. Something like this, may be:
    >

    I think the bool flag approach is much better. No assumption on the
    node initialization.

    If we go with bool approach then add

    Reviewed-by: Shakeel Butt <shakeelb@google.com>

    > 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..8e605e40a4c6 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 +447,7 @@ 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;
    >

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