linux-kernel - Re: [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink

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Message-ID: <20190529162638.GB3228@tower.DHCP.thefacebook.com>
Date:   Wed, 29 May 2019 16:26:43 +0000
From:   Roman Gushchin <guro@...com>
To:     Uladzislau Rezki <urezki@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink_va()

On Wed, May 29, 2019 at 03:58:17PM +0200, Uladzislau Rezki wrote:
> Hello, Roman!
> 
> > > Move the BUG_ON()/RB_EMPTY_NODE() check under unlink_va()
> > > function, it means if an empty node gets freed it is a BUG
> > > thus is considered as faulty behaviour.
> > 
> > It's not exactly clear from the description, why it's better.
> > 
> It is rather about if "unlink" happens on unhandled node it is
> faulty behavior. Something that clearly written in stone. We used
> to call "unlink" on detached node during merge, but after:
> 
> [PATCH v3 3/4] mm/vmap: get rid of one single unlink_va() when merge
> 
> it is not supposed to be ever happened across the logic.
> 
> >
> > Also, do we really need a BUG_ON() in either place?
> > 
> Historically we used to have the BUG_ON there. We can get rid of it
> for sure. But in this case, it would be harder to find a head or tail
> of it when the crash occurs, soon or later.
> 
> > Isn't something like this better?
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index c42872ed82ac..2df0e86d6aff 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1118,7 +1118,8 @@ EXPORT_SYMBOL_GPL(unregister_vmap_purge_notifier);
> >  
> >  static void __free_vmap_area(struct vmap_area *va)
> >  {
> > -       BUG_ON(RB_EMPTY_NODE(&va->rb_node));
> > +       if (WARN_ON_ONCE(RB_EMPTY_NODE(&va->rb_node)))
> > +               return;
> >
> I was thinking about WARN_ON_ONCE. The concern was about if the
> message gets lost due to kernel ring buffer. Therefore i used that.
> I am not sure if we have something like WARN_ONE_RATELIMIT that
> would be the best i think. At least it would indicate if a warning
> happens periodically or not.
> 
> Any thoughts?

Hello, Uladzislau!

I don't have a strong opinion here. If you're worried about losing the message,
WARN_ON() should be fine here. I don't think that this event will happen often,
if at all.

Thanks!