Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address bits PMD and PTE levels

[Steven Noonan]

On Tue, Apr 8, 2014 at 8:16 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> <snark>
>
> Of course, it would also be preferable if Amazon (or anything else) didn't need Xen PV :(

Well Amazon doesn't expose NUMA on PV, only on HVM guests.

> On April 7, 2014 9:04:53 PM PDT, Steven Noonan <steven@xxxxxxxxxxxxxx> wrote:
>>On Mon, Apr 7, 2014 at 2:25 PM, Mel Gorman <mgorman@xxxxxxx> 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
>>>> _PAGE_IOMAP)
>>>>
>>>> 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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