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Date: Fri, 27 Oct 2017 10:25:36 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Craig Bergstrom <craigb@google.com>,
        Sander Eikelenboom <linux@eikelenboom.it>,
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Subject: Re: ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem to valid physical
 addresses"): kernel BUG at arch/x86/mm/physaddr.c:79!
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, Oct 26, 2017 at 9:02 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > Well, 'mem=2048M' shouldn't really limit device memory, it's supposed to limit
> > (trim) 'RAM' and not much else.
> 
> Agreed. You should very much be able to map in IO memory or whatever
> above the 2G address even if the high_memory itself might be limited
> to 2GB.
> 
> So I think that commit ce56a86e2ade ("x86/mm: Limit mmap() of /dev/mem
> to valid physical addresses") is wrong, in that "high_memory" is very
> much the wrong thing to test.
> 
> The memory mapping limit might validly be something like
> 
>    1ull << boot_cpu_data.x86_phys_bits
> 
> or similar, but for now I suspect that the right thing to do is to
> revert. I'm not convinced that our "x86_phys_bits" value is guaranteed
> to be always right, since I think we mainlyjust use it for showing
> things, rather than have lots of code that depends on it.
> 
> Ingo?

Yeah, I think a more robust condition would be something like:

int valid_phys_addr_range(phys_addr_t addr, size_t count)
{
	return !((addr + count) >> MAX_PHYSMEM_BITS);
}

... as we already rely on MAX_PHYSMEM_BITS in a number of other critical places.

(Totally untested though.)

Thanks,

	Ingo