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From: Uladzislau Rezki <urezki@gmail.com>
To: Roman Gushchin <guro@fb.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Hillf Danton <hdanton@sina.com>, Michal Hocko <mhocko@suse.com>,
	Matthew Wilcox <willy@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Garnier <thgarnie@google.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joel Fernandes <joelaf@google.com>,
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink_va()
Date: Wed, 29 May 2019 15:58:17 +0200	[thread overview]
Message-ID: <20190529135817.tr7usoi2xwx5zl2s@pc636> (raw)
In-Reply-To: <20190528225001.GI27847@tower.DHCP.thefacebook.com>

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

  reply	other threads:[~2019-05-29 13:58 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-27  9:38 [PATCH v3 0/4] Some cleanups for the KVA/vmalloc Uladzislau Rezki (Sony)
2019-05-27  9:38 ` [PATCH v3 1/4] mm/vmap: remove "node" argument Uladzislau Rezki (Sony)
2019-05-28 22:33   ` Roman Gushchin
2019-05-27  9:38 ` [PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose Uladzislau Rezki (Sony)
2019-05-28 22:42   ` Roman Gushchin
2019-05-29 14:27     ` Uladzislau Rezki
2019-05-29 16:34       ` Roman Gushchin
2019-06-03 17:53         ` Uladzislau Rezki
2019-06-03 20:53           ` Uladzislau Rezki
2019-06-03 21:06             ` Roman Gushchin
2019-05-27  9:38 ` [PATCH v3 3/4] mm/vmap: get rid of one single unlink_va() when merge Uladzislau Rezki (Sony)
2019-05-28 22:45   ` Roman Gushchin
2019-05-27  9:38 ` [PATCH v3 4/4] mm/vmap: move BUG_ON() check to the unlink_va() Uladzislau Rezki (Sony)
2019-05-27 12:59   ` Steven Rostedt
2019-05-27 14:02     ` Uladzislau Rezki
2019-05-28 22:50   ` Roman Gushchin
2019-05-29 13:58     ` Uladzislau Rezki [this message]
2019-05-29 16:26       ` Roman Gushchin
2019-06-03 17:35         ` Uladzislau Rezki
2019-06-03 20:30           ` Roman Gushchin

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