linux-kernel - Re: [PATCH 4/7] kvm mmu: implement necessary data structures for second huge page accounting

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Date:	Sun, 29 Mar 2009 16:26:46 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joerg.roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] kvm mmu: implement necessary data structures for
 second huge page accounting

Joerg Roedel wrote:
> This patch adds the necessary data structures to take care of write
> protections in place within a second huge page sized page.
>
>
> +#ifdef KVM_PAGES_PER_LHPAGE
> +	if (npages && !new.hpage_info) {
> +		int hugepages = npages / KVM_PAGES_PER_LHPAGE;
> +		if (npages % KVM_PAGES_PER_LHPAGE)
> +			hugepages++;
> +		if (base_gfn % KVM_PAGES_PER_LHPAGE)
> +			hugepages++;
>   

Consider a slot with base_gfn == 1 and npages == 1.  This will have 
hugepages == 2, which is wrong.

I think the right calculation is

  ((base_gfn + npages - 1) / N) - (base_gfn / N) + 1

i.e. index of last page, plus one so we can store it.

The small huge page calculation is off as well.

> +
> +		new.hpage_info = vmalloc(hugepages * sizeof(*new.hpage_info));
> +
> +		if (!new.hpage_info)
> +			goto out_free;
> +
> +		memset(new.hpage_info, 0, hugepages * sizeof(*new.hpage_info));
> +
> +		if (base_gfn % KVM_PAGES_PER_LHPAGE)
> +			new.hpage_info[0].write_count = 1;
> +		if ((base_gfn+npages) % KVM_PAGES_PER_LHPAGE)
> +			new.hpage_info[hugepages-1].write_count = 1;
> +	}
> +#endif
> +
>   

-- 
error compiling committee.c: too many arguments to function

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