linux-kernel - Re: [patch 071/149] ARM: 6166/1: Proper prefetch abort handling on pre-ARMv6

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Date:	Tue, 6 Jul 2010 23:58:15 +0100
From:	Russell King <rmk@....linux.org.uk>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, stable-review@...nel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Anfei Zhou <anfei.zhou@...il.com>,
	Alexander Shishkin <virtuoso@...nd.org>,
	Siarhei Siamashka <siarhei.siamashka@...ia.com>
Subject: Re: [patch 071/149] ARM: 6166/1: Proper prefetch abort handling on
	pre-ARMv6

On Tue, Jul 06, 2010 at 04:06:18PM +0300, Kirill A. Shutemov wrote:
> I've investigated the issue. It's reproducible if you try to jump to
> the megabyte next to section mapping.

Okay, this is specific to the way that OMAP sets up its mappings, which
is why it doesn't appear everywhere.

> On ARM one Linux PGD entry contains two hardware entry. But there is error
> in do_translation_fault(). It's always call pmd_none() check for the first
> entry of two, not for the entry corresponded to address. So in case if we
> try to jump the megabyte next to section mapping, we will have inifinity
> loop of translation faults.

Okay, now that we know _why_ it happens, I'm satisfied that the fix
previously committed will help this situation.

> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index 9634fe1..825b9da 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -406,7 +406,8 @@ do_translation_fault(unsigned long addr, unsigned int fsr,
>         pmd_k = pmd_offset(pgd_k, addr);
>         pmd   = pmd_offset(pgd, addr);
>  
> -       if (pmd_none(*pmd_k))
> +       index = (addr >> SECTION_SHIFT) & 1;
> +       if (pmd_none(pmd_k[index]))

I do think this is extremely obscure, and therefore requires a comment
to help people understand what is going on here and why.  Leaving it
in the commit log would be an invitation for this to be needlessly
cut'n'pasted.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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