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

[Kirill Tkhai]

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()).

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.
 
> 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;
>  }
>