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SubjectRe: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address bits PMD and PTE levels
On Mon, Apr 7, 2014 at 2:25 PM, Mel Gorman <> wrote:
> On Mon, Apr 07, 2014 at 12:42:40PM -0700, H. Peter Anvin wrote:
>> On 04/07/2014 12:36 PM, Cyrill Gorcunov wrote:
>> > On Mon, Apr 07, 2014 at 12:27:10PM -0700, H. Peter Anvin wrote:
>> >> On 04/07/2014 11:28 AM, Mel Gorman wrote:
>> >>>
>> >>> I had considered the soft-dirty tracking usage of the same bit. I thought I'd
>> >>> be able to swizzle around it or a further worst case of having soft-dirty and
>> >>> automatic NUMA balancing mutually exclusive. Unfortunately upon examination
>> >>> it's not obvious how to have both of them share a bit and I suspect any
>> >>> attempt to will break CRIU. In my current tree, NUMA_BALANCING cannot be
>> >>> set if MEM_SOFT_DIRTY which is not particularly satisfactory. Next on the
>> >>> list is examining if _PAGE_BIT_IOMAP can be used.
>> >>
>> >> Didn't we smoke the last user of _PAGE_BIT_IOMAP?
>> >
>> > Seems so, at least for non-kernel pages (not considering this bit references in
>> > xen code, which i simply don't know but i guess it's used for kernel pages only).
>> >
>> David Vrabel has a patchset which I presumed would be pulled through the
>> Xen tree this merge window:
>> [PATCHv5 0/8] x86/xen: fixes for mapping high MMIO regions (and remove
>> That frees up this bit.
> Thanks, I was not aware of that patch. Based on it, I intend to force
> automatic NUMA balancing to depend on !XEN and see what the reaction is. If
> support for Xen is really required then it potentially be re-enabled if/when
> that series is merged assuming they do not need the bit for something else.

Amazon EC2 does have large memory instance types with NUMA exposed to
the guest (e.g. c3.8xlarge, i2.8xlarge, etc), so it'd be preferable
(to me anyway) if we didn't require !XEN.

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