Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message

[Yang Shi]

On 9/30/17 4:00 AM, Tetsuo Handa wrote:
> Yang Shi wrote:
> > On 9/28/17 1:45 PM, Tetsuo Handa wrote:
> > > Yang Shi wrote:
> > > > On 9/28/17 12:57 PM, Tetsuo Handa wrote:
> > > > > Yang Shi wrote:
> > > > > > On 9/27/17 9:36 PM, Tetsuo Handa wrote:
> > > > > > > On 2017/09/28 6:46, Yang Shi wrote:
> > > > > > > > Changelog v7 -> v8:
> > > > > > > > * Adopted Michal’s suggestion to dump unreclaim slab info when unreclaimable slabs amount > total user memory. Not only in oom panic path.
> > > > > > >
> > > > > > > Holding slab_mutex inside dump_unreclaimable_slab() was refrained since V2
> > > > > > > because there are
> > > > > > >
> > > > > > > 	mutex_lock(&slab_mutex);
> > > > > > > 	kmalloc(GFP_KERNEL);
> > > > > > > 	mutex_unlock(&slab_mutex);
> > > > > > >
> > > > > > > users. If we call dump_unreclaimable_slab() for non OOM panic path, aren't we
> > > > > > > introducing a risk of crash (i.e. kernel panic) for regular OOM path?
> > > > > >
> > > > > > I don't see the difference between regular oom path and oom path other
> > > > > > than calling panic() at last.
> > > > > >
> > > > > > And, the slab dump may be called by panic path too, it is for both
> > > > > > regular and panic path.
> > > > >
> > > > > Calling a function that might cause kerneloops immediately before calling panic()
> > > > > would be tolerable, for the kernel will panic after all. But calling a function
> > > > > that might cause kerneloops when there is no plan to call panic() is a bug.
> > > >
> > > > I got your point. slab_mutex is used to protect the list of all the
> > > > slabs, since we are already in oom, there should be not kmem cache
> > > > destroy happen during the list traverse. And, list_for_each_entry() has
> > > > been replaced to list_for_each_entry_safe() to make the traverse more
> > > > robust.
> > >
> > > I consider that OOM event and kmem chache destroy event can run concurrently
> > > because slab_mutex is not held by OOM event (and unfortunately cannot be held
> > > due to possibility of deadlock) in order to protect the list of all the slabs.
> > >
> > > I don't think replacing list_for_each_entry() with list_for_each_entry_safe()
> > > makes the traverse more robust, for list_for_each_entry_safe() does not defer
> > > freeing of memory used by list element. Rather, replacing list_for_each_entry()
> > > with list_for_each_entry_rcu() (and making relevant changes such as
> > > rcu_read_lock()/rcu_read_unlock()/synchronize_rcu()) will make the traverse safe.
> >
> > I'm not sure if rcu could satisfy this case. rcu just can protect
> > slab_caches_to_rcu_destroy list, which is used by SLAB_TYPESAFE_BY_RCU
> > slabs.
>
> I'm not sure why you are talking about SLAB_TYPESAFE_BY_RCU.
> What I meant is that
>
>    Upon registration:
>
>      // do initialize/setup stuff here
>      synchronize_rcu(); // <= for dump_unreclaimable_slab()
>      list_add_rcu(&kmem_cache->list, &slab_caches);
>
>    Upon unregistration:
>
>      list_del_rcu(&kmem_cache->list);
>      synchronize_rcu(); // <= for dump_unreclaimable_slab()
>      // do finalize/cleanup stuff here
>
> then (if my understanding is correct)
>
> 	rcu_read_lock();
> 	list_for_each_entry_rcu(s, &slab_caches, list) {
> 		if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))
> 			continue;
>
> 		memset(&sinfo, 0, sizeof(sinfo));
> 		get_slabinfo(s, &sinfo);
>
> 		if (sinfo.num_objs > 0)
> 			pr_info("%-17s %10luKB %10luKB\n", cache_name(s),
> 				(sinfo.active_objs * s->size) / 1024,
> 				(sinfo.num_objs * s->size) / 1024);
> 	}
> 	rcu_read_unlock();
>
> will make dump_unreclaimable_slab() safe.

Thanks for the detailed description. However, it sounds this change is  
too much for slub, I'm not sure if this may change the subtle behavior  
of slub.

trylock sounds like a good alternative.

Yang

>