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

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Date:	Fri, 16 Nov 2012 18:55:17 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	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 Fri, Nov 16, 2012 at 07:04:04PM +0100, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@...e.de> wrote:
> 
> > That said, your approach just ends up being heavier. [...]
> 
> Well, it's more fundamental than just whether to inline or not 
> (which I think should be a separate optimization and I won't 
> object to two-instruction variants the slightest) - but you 
> ended up open-coding change_protection() 
> via:
> 
>    change_prot_numa_range() et al
> 
> which is a far bigger problem...
> 
> Do you have valid technical arguments in favor of that 
> duplication?
> 

No, I don't and I have not claimed that it *has* to exist. In fact I've
said multiple times than I can convert to change_protection as long as
_PAGE_NUMA == _PAGE_NONE. This initial step was to build the list
of requirements without worrying about breaking existing users of
change_protection. Now that I know what the requirements are, I can convert.

> If you just embrace the PROT_NONE reuse approach of numa/core 
> then 90% of the differences in your tree will disappear and 
> you'll have a code base very close to where numa/core was 3 
> weeks ago already, modulo a handful of renames.
> 

Pointed out the missing parts in another mail already -- MIGRATE_FAULT,
pmd handling in batch, stats and a logical progression from a simple to
a complex policy.

> It's not like PROT_NONE will go away anytime soon.
> 
> PROT_NONE is available on every architecture, and we use the 
> exact semantics of it in the scheduler, we just happen to drive 
> it from a special worklet instead of a syscall, and happen to 
> have a callback to the faults when they happen...
> 
> Please stay open to that approach.
> 

I will.

If anything, me switching to prot_none would be a hell of a lot easier
than you trying to pick up the bits you're missing. I'll take a look
Monday and see what falls out.

-- 
Mel Gorman
SUSE Labs
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