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Date:   Thu, 26 Oct 2017 21:29:19 +0200
From:   Linus Torvalds <>
To:     Ingo Molnar <>
Cc:     Craig Bergstrom <>,
        Sander Eikelenboom <>,
        Boris Ostrovsky <>,
        Fengguang Wu <>,,
        Linux Kernel Mailing List <>,
        LKP <>
Subject: Re: ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem to valid physical
 addresses"): kernel BUG at arch/x86/mm/physaddr.c:79!

On Thu, Oct 26, 2017 at 9:02 PM, Ingo Molnar <> 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.



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