linux-kernel - Re: unable to mmap /dev/kmem

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Date:	Fri, 19 Jan 2007 12:31:18 +0100
From:	"Franck Bui-Huu" <fbuihuu@...il.com>
To:	"Hugh Dickins" <hugh@...itas.com>
Cc:	"Nadia Derbey" <Nadia.Derbey@...l.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: unable to mmap /dev/kmem

Hi,

Hugh Dickins wrote:
> On Thu, 18 Jan 2007, Nadia Derbey wrote:
>> Trying to mmap /dev/kmem with an offset I take from /boot/System.map,
>> I get an EIO error on a 2.6.20-rc4.
>> This is something that used to work on older kernels.
>>
>> Had a look at mmap_kmem() in drivers/char/mem.c, and I'm wondering whether
>> pfn is correctly computed there: shouldn't we have something like
>>
>> pfn = PFN_DOWN(virt_to_phys((void *)PAGE_OFFSET)) +
>>       __pa(vma->vm_pgoff << PAGE_SHIFT);
>>
>> instead of
>>
>> pfn = PFN_DOWN(virt_to_phys((void *)PAGE_OFFSET)) + vma->vm_pgoff;
>>
>> Or may be should I substract PAGE_OFFSET from the value I get from System.map
>> before mmapping /dev/kmem?
>
> Sigh, you're right, 2.6.19 messed that up completely.
> No, you never had to subtract PAGE_OFFSET from that address
> in the past, and you shouldn't have to do so now.
>
> Please revert the offending patch below, and then maybe Franck
> can come up with a patch which preserves the original behaviour
> on architectures which used to work (e.g. i386), while fixing
> it for those architectures (which are they?) that did not.
>

I've been confused by 'vma->vm_pgoff' meaning. I assumed that it was
an offset relatif to the start of the kernel address space
(PAGE_OFFSET) as the commit message I submitted explains. So doing:

	fd = open("/dev/kmem", O_RDONLY);
	kmem = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 4 * 4096);

actually asks for a kernel space mapping which start 4 pages after the
begining of the kernel memory space.

So yes, if the 'offset' expected by 'mmap(/dev/kmem, ..., offset)'
usage is actually a kernel virtual address the patch I submitted is
wrong. The offending line should have been something like:

	pfn = PFN_DOWN(virt_to_phys((void *)(vma->vm_pgoff << PAGE_SHIFT)));

and in this case 'vma->vm_pgoff' has no sense to me. My apologizes for
this mess.

> I guess it's reassuring to know that not many are actually
> using mmap of /dev/kmem, so you're the first to notice: thanks.
>

yes it doesn't seems to be used. In my case, I was just playing with
it when I submitted the patch but have no real usages.

		Franck
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