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

[Yang Shi]

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.

Yang

>