Re: [PATCH] mm: list_lru: hold nlru lock to avoid reading transient negative nr_items

[Yang Shi]

On Tue, Dec 1, 2020 at 2:09 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> On 30.11.2020 21:45, Yang Shi wrote:
> > When investigating a slab cache bloat problem, significant amount of
> > negative dentry cache was seen, but confusingly they neither got shrunk
> > by reclaimer (the host has very tight memory) nor be shrunk by dropping
> > cache.  The vmcore shows there are over 14M negative dentry objects on lru,
> > but tracing result shows they were even not scanned at all.  The further
> > investigation shows the memcg's vfs shrinker_map bit is not set.  So the
> > reclaimer or dropping cache just skip calling vfs shrinker.  So we have
> > to reboot the hosts to get the memory back.
> >
> > I didn't manage to come up with a reproducer in test environment, and the
> > problem can't be reproduced after rebooting.  But it seems there is race
> > between shrinker map bit clear and reparenting by code inspection.  The
> > hypothesis is elaborated as below.
> >
> > The memcg hierarchy on our production environment looks like:
> >                 root
> >                /    \
> >           system   user
> >
> > The main workloads are running under user slice's children, and it creates
> > and removes memcg frequently.  So reparenting happens very often under user
> > slice, but no task is under user slice directly.
> >
> > So with the frequent reparenting and tight memory pressure, the below
> > hypothetical race condition may happen:
> >
> >     CPU A                            CPU B                         CPU C
> > reparent
> >     dst->nr_items == 0
> >                                  shrinker:
> >                                      total_objects == 0
> >     add src->nr_items to dst
> >     set_bit
> >                                      retrun SHRINK_EMPTY
> >                                      clear_bit
> >                                                                   list_lru_del()
> > reparent again
> >     dst->nr_items may go negative
> >     due to current list_lru_del()
> >     on CPU C
> >                                  The second run of shrinker:
> >                                      read nr_items without any
> >                                      synchronization, so it may
> >                                      see intermediate negative
> >                                      nr_items then total_objects
> >                                      may return 0 conincidently
> >
> >                                      keep the bit cleared
> >     dst->nr_items != 0
> >     skip set_bit
> >     add scr->nr_item to dst
>
> Good catch, Yang. Thanks for investigating this.
>
> But I agree with Roman it's better to fix that in rare-called place
> (memcg_drain_list_lru_node()), than in hot place (list_lru_count_one()).

Yes, agreed. Will incarnate Roman's proposal in v2.

>
> Also, I'd added to description of new patch a reference to memcg_offline_kmem(),
> because this is the place, where child->kmemcg_id is rewritten, and
> this is the reason of lru's nr_items may become negative.

Sure.

>
> > After this point dst->nr_item may never go zero, so reparenting will not
> > set shrinker_map bit anymore.  And since there is no task under user
> > slice directly, so no new object will be added to its lru to set the
> > shrinker map bit either.  That bit is kept cleared forever.
> >
> > How does list_lru_del() race with reparenting?  It is because
> > reparenting replaces childen's kmemcg_id to parent's without protecting
> > from nlru->lock, so list_lru_del() may see parent's kmemcg_id but
> > actually deleting items from child's lru, but dec'ing parent's nr_items,
> > so the parent's nr_items may go negative as commit
> > 2788cf0c401c268b4819c5407493a8769b7007aa ("memcg: reparent list_lrus and
> > free kmemcg_id on css offline") says.
> >
> > Can we move kmemcg_id replacement after reparenting?  No, because the
> > race with list_lru_del() may result in negative src->nr_items, but it
> > will never be fixed.  So the shrinker may never return SHRINK_EMPTY then
> > keep the shrinker map bit set always.  The shrinker will be always
> > called for nonsense.
> >
> > Can we synchronize list_lru_del() and reparenting?  Yes, it could be
> > done.  But it seems we need introduce a new lock or use nlru->lock.  But
> > it sounds complicated to move kmemcg_id replacement code under nlru->lock.
> > And list_lru_del() may be called quite often to exacerbate some hot
> > path, i.e. dentry kill.
> >
> > So, it sounds acceptable to synchronize reading nr_items to avoid seeing
> > intermediate negative nr_items given the simplicity and it is typically
> > just called by shrinkers when counting the freeable objects.
> >
> > The patch is tested with some shrinker intensive workloads, no
> > noticeable regression is soptted.
> >
> > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> > Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > Cc: Roman Gushchin <guro@xxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> >  mm/list_lru.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index 5aa6e44bc2ae..5c128a7710ff 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -178,10 +178,17 @@ unsigned long list_lru_count_one(struct list_lru *lru,
> >       struct list_lru_one *l;
> >       unsigned long count;
> >
> > -     rcu_read_lock();
> > +     /*
> > +      * Since list_lru_{add,del} may be called under an IRQ-safe lock,
> > +      * we have to use IRQ-safe primitives here to avoid deadlock.
> > +      *
> > +      * Hold the lock to prevent from seeing transient negative
> > +      * nr_items value.
> > +      */
> > +     spin_lock_irq(&nlru->lock);
> >       l = list_lru_from_memcg_idx(nlru, memcg_cache_id(memcg));
> >       count = READ_ONCE(l->nr_items);
> > -     rcu_read_unlock();
> > +     spin_unlock_irq(&nlru->lock);
> >
> >       return count;
> >  }
> >
>
>