linux-kernel - 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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Message-ID: <CA+55aFw_7PFDnTRO1C6gH99vdY+w77Rp_8Ns8krSjxmxSs6mVw@mail.gmail.com>
Date:   Thu, 26 Oct 2017 21:29:19 +0200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Craig Bergstrom <craigb@...gle.com>,
        Sander Eikelenboom <linux@...elenboom.it>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Fengguang Wu <fengguang.wu@...el.com>, wfg@...ux.intel.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LKP <lkp@...org>
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 <mingo@...nel.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?

               Linus