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SubjectRe: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for _PAGE_NUMA v2

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Apr 08, 2014 at 05:46:52PM +0100, Mel Gorman wrote:
> > Someone will ask why automatic NUMA balancing hints do not use "real"
> > PROT_NONE but as it would need VMA information to do that on all
> > architectures it would mean that VMA-fixups would be required when marking
> > PTEs for NUMA hinting faults so would be expensive.
>
> Like this:
>
> https://lkml.org/lkml/2012/11/13/431
>
> That used the generic PROT_NONE infrastructure and compared, on fault,
> the page protection bits against the vma->vm_page_prot bits?
>
> So the objection to that approach was the vma-> dereference in
> pte_numa() ?

I think the real underlying objection was that PTE_NUMA was the last
leftover from AutoNUMA, and removing it would have made it not a
'compromise' patch set between 'AutoNUMA' and 'sched/numa', but would
have made the sched/numa approach 'win' by and large.

The whole 'losing face' annoyance that plagues all of us (me
included).

I didn't feel it was important to the general logic of adding access
pattern aware NUMA placement logic to the scheduler, and I obviously
could not ignore the NAKs from various mm folks insisting on PTE_NUMA,
so I conceded that point and Mel built on that approach as well.

Nice it's being cleaned up, and I'm pretty happy about how NUMA
balancing ended up looking like.

Thanks,

Ingo


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