linux-kernel - Re: [PATCH] [1/9] Handle kernel near memory hole in clear_kernel

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Message-ID: <20080131162220.GA25989@bingen.suse.de>
Date:	Thu, 31 Jan 2008 17:22:20 +0100
From:	Andi Kleen <ak@...e.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <ak@...e.de>, ebiederm@...ssion.com, vgoyal@...hat.com,
	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [1/9] Handle kernel near memory hole in
	clear_kernel_mapping

> > +#define overlaps(as, ae, bs, be) ((ae) >= (bs) && (as) <= (be))
> 
> inline function please and a bit more intuituive arrangement of
> arguments.

Which one do you prefer? (to be honest the current one is "intuitive"
to me) 

> 
> > +void __init clear_kernel_mapping(unsigned long address, unsigned long size)
> > +{
> > +	int sh = PMD_SHIFT;
> > +	unsigned long kernel = __pa(__START_KERNEL_map);
> > +
> > +	/*
> > +	 * Note that we cannot unmap the kernel itself because the unmapped
> > +	 * holes here are always at least 2MB aligned.
> 
> That's not enforced. The unmap code just does not split pages.

It is -- there are BUG_ONs for this in __clear_kernel_mapping

> 
> > +	 * This just applies to the trailing areas of the 40MB kernel mapping.
> 
> How is this ensured, that it only affects the end of the 40MB mapping ?

It is enforced in the callers (actually there is only a single caller -- 
the GART code ) by not calling it overlapping for the kernel itself.
Given that could be checked too, but that would be probably overkill
for an internal function.

> 
> > +	 */
> > +	if (overlaps(kernel >> sh, (kernel + KERNEL_TEXT_SIZE) >> sh,
> > +			__pa(address) >> sh, __pa(address + size) >> sh)) {
> 
> This checks:
> 
>      (kernel_end + 1) >= gart_start && kernel_start <= gart_end
> 
> One off error:  kernel + KERNEL_TEXT_SIZE 
> needs to be:    kernel + KERNEL_TEXT_SIZE - 1

Ok.

> 
> Also there is no sanity check, whether the area is inside real kernel text.

Hmm I can add one, but if that happens the caller is likely seriously
confused and will likely cause other problems anyways. 

I don't think it can happen for the GART code which is currently
the only caller.

> 
> > +		printk(KERN_WARNING
> > +			"Kernel mapping at %lx within 2MB of memory hole\n",
> > +				kernel);
> 
> 
> > +		__clear_kernel_mapping(__START_KERNEL_map+__pa(address), size);
> 
> Doh! This is unmapping the wrong place. According to __phys_addr():
> 
>      paddr = vaddr - __START_KERNEL_map + phys_base;

Hmm true -- that will only affect relocatable kernels, but for those
it's wrong. Given that the patch was supposed to fix a case 
that only happens relocatable kernels that's quite ironic :)

Actually thinking about it again you can just drop it for now.
It is orthogonal to gbpages. I think I added it to the series
when I was planning to do kernel mapping as GB pages, but that
turned out to be a bad idea anyways.

Thanks for the review,

-Andi
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