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

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Mon, Apr 07, 2014 at 07:28:54PM +0100, Mel Gorman wrote:
> > > I didn't bother spelling it out in case I gave the impression that I was
> > > blaming Xen for the problem.  As the bit is now changes, does it help
> > > the Xen problem or cause another collision of some sort? There is no
> > > guarantee _PAGE_NUMA will remain as bit 62 but at worst it'll use bit 11
> > > and NUMA_BALANCING will depend in !KMEMCHECK.
> > 
> > Fwiw, we're using bit 11 for soft-dirty tracking, so i really hope worst case
> > never happen. (At the moment I'm trying to figure out if with this set
> > it would be possible to clean up ugly macros in pgoff_to_pte for 2 level pages).
> 
> 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.

Thanks for info, Mel! It seems indeed if no more space left on x86-64 (in
the very worst case which I still think won't happen anytime soon) we'll
have to make them mut. exclusive. But for now (with 62 bit used for numa)
they can live together, right?