linux-kernel - Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline

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Date:	Wed, 31 Jul 2013 09:55:59 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
CC:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

On 07/29/2013 11:49 PM, Xishi Qiu wrote:
> I think we can remove "BUG_ON(start_pfn >= end_pfn)" in __offline_pages(),
> because in memory_block_action() "nr_pages = PAGES_PER_SECTION * sections_per_block" 
> is always greater than 0.
...
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1472,7 +1472,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	struct zone *zone;
>  	struct memory_notify arg;
>  
> -	BUG_ON(start_pfn >= end_pfn);
>  	/* at least, alignment against pageblock is necessary */
>  	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
>  		return -EINVAL;

I think you're saying that you don't see a way to hit this BUG_ON() in
practice.  That does appear to be true, unless sections_per_block ended
up 0 or negative.  The odds of getting in to this code if
'sections_per_block' was bogus are pretty small.

Or, is this a theoretical thing that folks might run in to when adding
new features or developing?  It's in a cold path and the cost of the
check is miniscule.  The original author (cc'd) also saw a need to put
this in probably because he actually ran in to this.

In any case, it looks fairly safe to me:

Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
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