linux-kernel - Re: [PATCH 1/2] mm: page_isolation: check pfn validity before access

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Message-ID: <20141112193450.GA18936@dhcp22.suse.cz>
Date:	Wed, 12 Nov 2014 20:34:50 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Weijie Yang <weijie.yang@...sung.com>
Cc:	kamezawa.hiroyu@...fujitsu.com, 'Minchan Kim' <minchan@...nel.org>,
	'Andrew Morton' <akpm@...ux-foundation.org>, mgorman@...e.de,
	mina86@...a86.com, 'linux-kernel' <linux-kernel@...r.kernel.org>,
	'Linux-MM' <linux-mm@...ck.org>,
	'Weijie Yang' <weijie.yang.kh@...il.com>
Subject: Re: [PATCH 1/2] mm: page_isolation: check pfn validity before access

On Thu 06-11-14 16:08:02, Weijie Yang wrote:
> In the undo path of start_isolate_page_range(), we need to check
> the pfn validity before access its page, or it will trigger an
> addressing exception if there is hole in the zone.

This looks a bit fishy to me. I am not familiar with the code much but
at least __offline_pages zone = page_zone(pfn_to_page(start_pfn)) so it
would blow up before we got here. Same applies to the other caller
alloc_contig_range. So either both need a fix and then
start_isolate_page_range doesn't need more checks or this is all
unnecessary.

Please do not make this code more obfuscated than it is already...

> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
> ---
>  mm/page_isolation.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index d1473b2..3ddc8b3 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -137,8 +137,11 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  undo:
>  	for (pfn = start_pfn;
>  	     pfn < undo_pfn;
> -	     pfn += pageblock_nr_pages)
> -		unset_migratetype_isolate(pfn_to_page(pfn), migratetype);
> +	     pfn += pageblock_nr_pages) {
> +		page = __first_valid_page(pfn, pageblock_nr_pages);
> +		if (page)
> +			unset_migratetype_isolate(page, migratetype);
> +	}
>  
>  	return -EBUSY;
>  }
> -- 
> 1.7.0.4
> 
> 
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-- 
Michal Hocko
SUSE Labs
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