Re: [PATCH] mm/hotplug: remove unnecessary BUG_ON in __offline_pages()

From: Dave Hansen
Date: Wed Jul 31 2013 - 12:56:21 EST

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@xxxxxxxxxxxxxxx>
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