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    SubjectRe: ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem to valid physical addresses"): kernel BUG at arch/x86/mm/physaddr.c:79!
    Reverting seems like the right approach at the moment.  My apologies
    for the breakage so late the in the cycle.

    Post-revert, there remains a bug here wherein you can make the system
    OOPS if you mmap memory above the 48 bit bus width. Linus/Ingo, is
    there something in particular that you'd like to see before pulling in
    a check on the bus width (or some other fix)? Based on Linus'
    comment, the bus width check smells like something that should cook
    for a while before pulling into mainline.

    On Thu, Oct 26, 2017 at 1:29 PM, 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?
    >
    > Linus

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