Re: [PATCH RFC] x86,mm: use _PAGE_BIT_SOFTW2 as _PAGE_BIT_NUMA

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Nov 07, 2014 at 08:52:10AM +0000, Mel Gorman wrote:
> On Thu, Nov 06, 2014 at 05:48:16PM +0000, Wei Liu wrote:
> > In b38af4721 ("x86,mm: fix pte_special versus pte_numa") pte_special()
> > (SPECIAL with PRESENT or PROTNONE) was made to complement pte_numa()
> > (SPECIAL with neither PRESENT nor PROTNONE). That broke Xen PV guest
> > with NUMA balancing support.
> > 
> > That's because Xen hypervisor sets _PAGE_GLOBAL (_PAGE_GLOBAL /
> > _PAGE_PROTNONE in Linux) for guest user space mapping. So in a Xen PV
> > guest, when NUMA balancing is enabled, a NUMA hinted PTE ends up
> > "SPECIAL (in fact NUMA) with PROTNONE but not PRESENT", which makes
> > pte_special() returns true when it shouldn't.
> > 
> > Fundamentally we only need _PAGE_NUMA and _PAGE_PRESENT to tell
> > difference between an unmapped entry and an entry protected for NUMA
> > hinting fault. So use _PAGE_BIT_SOFTW2 as _PAGE_BIT_NUMA, adjust
> > _PAGE_NUMA_MASK and SWP_OFFSET_SHIFT as needed.
> > 
> > Suggested-by: David Vrabel <david.vrabel@citrix.com>
> > Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> 
> I suggest instead that you force automatic NUMA balancing to be disabled
> on Xen PV guests until I or someone else finds time to implement Linus'
> idea to remove _PAGE_NUMA entirely. It's been on my TODO list for a few
> weeks but I still have not reached the point where I'm back working on
> upstream material properly.
>  

No problem. Thanks for the suggestion.

Wei.

> -- 
> Mel Gorman
> SUSE Labs

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