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

[Mel Gorman]

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.

-- 
Mel Gorman
SUSE Labs
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