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

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Date:   Mon, 27 May 2019 08:59:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Roman Gushchin <guro@...com>, 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>,
        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 Mon, 27 May 2019 11:38:42 +0200
"Uladzislau Rezki (Sony)" <urezki@...il.com> wrote:

> 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.

Can we switch it to a WARN_ON(). We are trying to remove all BUG_ON()s.
If a user wants to crash on warning, there's a sysctl for that. But
crashing the system can make it hard to debug. Especially if it is hit
by someone without a serial console, and the machine just hangs in X.
That is very annoying.

With a WARN_ON, you at least get a chance to see the crash dump.

-- Steve


> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  mm/vmalloc.c | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 371aba9a4bf1..340959b81228 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -533,20 +533,16 @@ link_va(struct vmap_area *va, struct rb_root *root,
>  static __always_inline void
>  unlink_va(struct vmap_area *va, struct rb_root *root)
>  {
> -	/*
> -	 * During merging a VA node can be empty, therefore
> -	 * not linked with the tree nor list. Just check it.
> -	 */
> -	if (!RB_EMPTY_NODE(&va->rb_node)) {
> -		if (root == &free_vmap_area_root)
> -			rb_erase_augmented(&va->rb_node,
> -				root, &free_vmap_area_rb_augment_cb);
> -		else
> -			rb_erase(&va->rb_node, root);
> +	BUG_ON(RB_EMPTY_NODE(&va->rb_node));
>  
> -		list_del(&va->list);
> -		RB_CLEAR_NODE(&va->rb_node);
> -	}
> +	if (root == &free_vmap_area_root)
> +		rb_erase_augmented(&va->rb_node,
> +			root, &free_vmap_area_rb_augment_cb);
> +	else
> +		rb_erase(&va->rb_node, root);
> +
> +	list_del(&va->list);
> +	RB_CLEAR_NODE(&va->rb_node);
>  }
>  
>  #if DEBUG_AUGMENT_PROPAGATE_CHECK
> @@ -1187,8 +1183,6 @@ EXPORT_SYMBOL_GPL(unregister_vmap_purge_notifier);
>  
>  static void __free_vmap_area(struct vmap_area *va)
>  {
> -	BUG_ON(RB_EMPTY_NODE(&va->rb_node));
> -
>  	/*
>  	 * Remove from the busy tree/list.
>  	 */