Re: unable to mmap /dev/kmem

["Franck Bui-Huu" <fbuihuu@gmail.com>]

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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