linux-kernel - Re: [PATCH] x86: fix virt_addr_valid() with CONFIG_DEBUG

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Date:	Thu, 2 Oct 2008 08:18:03 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Jiri Slaby" <jirislaby@...il.com>
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Vegard Nossum" <vegardno@....uio.no>,
	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: fix virt_addr_valid() with CONFIG_DEBUG_VIRTUAL=y

On Wed, Oct 1, 2008 at 10:01 PM, Jiri Slaby <jirislaby@...il.com> wrote:
> On 10/01/2008 09:46 PM, Andi Kleen wrote:
>>> Anyway virt_addr_valid() is IMHO wrong. E.g. first modules VM address
>>> 0xffffffffa0000000 is after __pa() 200M which is valid pfn after the shift even
>>> on the flatmem model with enough memory.
>>>
>>> Am I missing something? What's the exact purpose of the virt_addr_valid()?
>>
>> I think it's supposed to be only used on direct mapping anyways (judging
>> from a quick look a the users)
>
> Then kmemcheck assumes something else. Citing:
>  * We need to be extremely careful not to follow any invalid pointers,
>  * because this function can be called for *any* possible address.
> and the very first check is !virt_addr_valid(address).

The purpose of this call was to make sure that the page behind the
virtual address has an associated struct page. That is the assumption:
virt_to_page() will return something meaningful if and only if
virt_addr_valid().

>
>> So not handling text mapping is ok, but don't panic on it.
>
> It doesn't handle properly anything but text and direct mapping. Now it
> oopses/causes BUG on that wrong cases.
>
> I think we should set it down there that it was intended to be used only on
> text/direct mapping and only for checking if there is a physical memory page
> behind this kind of virtual address.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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