linux-kernel - Re: [regression] Boot crash with: f7426b983a6a ("mm: cma: adjust address limit to avoid hitting low/high memory boundary")

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Date:	Wed, 10 Dec 2014 16:17:24 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Russell King <rmk@....linux.org.uk>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Drake <drake@...lessm.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [regression] Boot crash with: f7426b983a6a ("mm: cma: adjust
 address limit to avoid hitting low/high memory boundary")

On Tue, Dec 09, 2014 at 08:50:32AM +0000, Russell King wrote:
> On Tue, Dec 09, 2014 at 04:55:42PM +0900, Joonsoo Kim wrote:
> > Could you manage this fix for above boot regression in x86?
> > Patch itself is so dirty, because __pa_nodebug() is implemented only
> > in x86. If someone knows better idea, please let me know.
> 
> > +#ifdef CONFIG_X86
> > +	/*
> > +	 * high_memory isn't direct mapped memory so retrieving it's
> > +	 * physical address isn't appropriate. But, it would be useful
> > +	 * to check physical address of highmem boundary so it's
> > +	 * justfiable to get physical address from it. In x86, there is
> > +	 * a validation check for this case, so following workaround is
> > +	 * needed to avoid it.
> > +	 */
> > +	highmem_start = __pa_nodebug(high_memory);
> > +#else
> > +	highmem_start = __pa(high_memory);
> > +#endif
> 
> What about:
> 
> 	highmem_start = __pa(high_memory - 1) + 1;
> 
> As "high_memory - 1" should be a valid lowmem address.

Thanks for suggestion.
It looks tricky but it would work. I think that this one need to be
fully tested, so the first dirty one is better for the stable tree.

I will check and test Russell's suggestion and send it as clean-up.

Thanks.
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