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

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Message-ID: <20190529135817.tr7usoi2xwx5zl2s@pc636>
Date:   Wed, 29 May 2019 15:58:17 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Roman Gushchin <guro@...com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        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()

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?

Thanks for the comments!

--
Vlad Rezki