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

["H. Peter Anvin" <hpa@zytor.com>]

<snark>

Of course, it would also be preferable if Amazon (or anything else) didn't need Xen PV :(

On April 7, 2014 9:04:53 PM PDT, Steven Noonan <steven@uplinklabs.net> wrote:
>On Mon, Apr 7, 2014 at 2:25 PM, Mel Gorman <mgorman@suse.de> 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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