linux-kernel - Re: [PATCH 06/43] mm: numa: Make pte_numa() and pmd

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Date:	Fri, 16 Nov 2012 12:26:41 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/43] mm: numa: Make pte_numa() and pmd_numa() a generic
 implementation

On 11/16/2012 11:56 AM, Mel Gorman wrote:

>> b33467764d8a mm/migrate: Introduce migrate_misplaced_page()
>
> bolts onto the side of migration and introduces MIGRATE_FAULT which
> should not have been necessary. Already complained about.
>
> The alternative uses the existing migrate_pages() function but has
> different requirements for taking a reference to the page.

Indeed, NACK to b33467764d8a

Mel's tree implements this in a much cleaner way.

>> ca2ea0747a5b mm/mpol: Add MPOL_MF_LAZY
>
> We more or less share this except I backed out the userspace visible bits
> in a separate patch because I didn't think it had been carefully reviewed
> how an application should use it and if it was a good idea. Covered in an
> earlier review.

Agreed, these bits should not be userspace visible, at least
not for now.

>> cd203e33c39d mm/mpol: Add MPOL_MF_NOOP
>
> I have a patch that backs this out on the grounds that I don't think we
> have adequately discussed if it was the correct userspace interface. I
> know Peter put a lot of time into it so it's probably correct but
> without man pages or spending time writing an example program that used
> it, I played safe.

Ditto.

>> 6fe64360a759 mm: Only flush the TLB when clearing an accessible pte
>
> I missed this. Stupid stupid stupid! It would reduce the TLB flushes from
> migration context.

However, Ingo's tree still incurs the double page fault for
migrated pages. Both trees could use a little improvement in
this area :)

>> e9df40bfeb25 x86/mm: Introduce pte_accessible()
>
> prot_none.

This one is x86 specific, and would work as well with Andrea's
_PAGE_NUMA as it would with _PAGE_PROTNONE.

>> is a good foundation already with no WIP policy bits in it.
>>
>> Mel, could you please work on this basis, or point out the bits
>> you don't agree with so I can fix it?
>>
>
> My main hangup is the prot_none choice and I know it's something we have
> butted heads on without progress. I feel it is a lot cleaner to have
> the _PAGE_NUMA bit (even if it's PROT_NONE underneath) and the helpers
> avoid function calls where possible.

I am pretty neutral on whether we use _PAGE_NUMA with _PAGE_PROTNONE
underneath, or the slightly higher overhead actual prot_none stuff.

I can live with whichever of these Linus ends up merging.

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