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Date:   Thu, 26 Oct 2017 21:29:07 +0200
From:   Sander Eikelenboom <>
To:     Craig Bergstrom <>, Ingo Molnar <>
Cc:     Boris Ostrovsky <>,
        Fengguang Wu <>,,, LKP <>,
        Linus Torvalds <>,
        Xen-devel <>
Subject: Re: ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem to valid physical
 addresses"): kernel BUG at arch/x86/mm/physaddr.c:79!

On 26/10/17 19:49, Craig Bergstrom wrote:
> Sander, thanks for the details, they've been very useful.
> I suspect that your host system's mem=2048M parameter is causing the
> problem.  Any chance you can confirm by removing the parameter and
> running the guest code path?

I removed it, but kept the hypervisor limiting dom0 memory to 2046M intact (in grub using the xen bootcmd: 
"multiboot       /xen-4.10.gz  dom0_mem=2048M,max:2048M ....."

Unfortunately that doesn't change anything, the guest still fails to start with the same errors.

> More specifically, since you're telling the kernel that it's high
> memory address is at 2048M and your device is at 0xfe1fe000 (~4G), the
> new mmap() limits are preventing you from mapping addresses that are
> explicitly disallowed by the parameter.

Which would probably mean the current patch prohibits hard limiting the dom0 memory to a certain value (below 4G)
at least in combination with PCI-passthrough. So the only thing left would be to have no hard memory restriction on dom0
and rely on auto-ballooning, but I'm not a great fan of that.

I don't know how KVM handles setting memory limits for the host system, but perhaps it suffers from the same issue.

I also tried the patch from one of your last mails to make the check "less strict", 
but still get the same errors (when using the hard memory limits).


> On Thu, Oct 26, 2017 at 10:39 AM, Ingo Molnar <> wrote:
>> * Craig Bergstrom <> wrote:
>>> Yes, not much time left for 4.14, it might be reasonable to pull the
>>> change out since it's causing problems. [...]
>> Ok, I'll queue up a revert tomorrow morning and send it to Linus ASAP if there's
>> no good fix by then. In hindsight I should have queued it for v4.15 ...
>> Thanks,
>>         Ingo

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