Re: [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink_va()

Roman Gushchin
2019-05-29T16:28:21Z

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!