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

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Date:   Sat, 30 Sep 2017 20:00:04 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     yang.s@...baba-inc.com
Cc:     mhocko@...nel.org, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message

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.