Re: [PATCH 08/19] mm: numa: Create basic numa page hinting infrastructure

[Mel Gorman <mgorman@suse.de>]

On Tue, Nov 13, 2012 at 11:21:20AM +0100, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@suse.de> wrote:
> 
> > Note: This patch started as "mm/mpol: Create special PROT_NONE
> > 	infrastructure" and preserves the basic idea but steals *very*
> > 	heavily from "autonuma: numa hinting page faults entry points" for
> > 	the actual fault handlers without the migration parts.	The end
> > 	result is barely recognisable as either patch so all Signed-off
> > 	and Reviewed-bys are dropped. If Peter, Ingo and Andrea are ok with
> > 	this version, I will re-add the signed-offs-by to reflect the history.
> 
> Most of the changes you had to do here relates to the earlier 
> decision to turn it all the NUMA protection fault demultiplexing 
> and setup code into a per arch facility.
> 

Yes.

> On one hand I'm 100% fine with making the decision to *use* the 
> new NUMA code per arch and explicitly opt-in - we already have 
> such a Kconfig switch in our tree already. The decision whether 
> to use any of this for an architecture must be considered and 
> tested carefully.
> 

Agreed.

> But given that most architectures will be just fine reusing the 
> already existing generic PROT_NONE machinery, the far better 
> approach is to do what we've been doing in generic kernel code 
> for the last 10 years: offer a default generic version, and then 
> to offer per arch hooks on a strict as-needed basis, if they 
> want or need to do something weird ...
> 

If they are *not* fine with it, it's a large retrofit because the PROT_NONE
machinery has been hard-coded throughout. It also requires that anyone
looking at the fault paths must remember at almost all times that PROT_NONE
can also mean PROT_NUMA and it depends on context. While that's fine right
now, it'll be harder to maintain in the future.

> So why fork away this logic into per arch code so early and 
> without explicit justification? It creates duplication artifacts 
> all around and makes porting to a new 'sane' architecture 
> harder.
> 

I agree that the duplication artifacts was a mistake. I can fix that but
feel that the naming is fine and we shouldn't hard-code that
change_prot_none() can actually mean change_prot_numa() if called from
the right place.

> Also, if there *are* per architecture concerns then I'd very 
> much like to see that argued very explicitly, on a per arch 
> basis, as it occurs, not obscured through thick "just in case" 
> layers of abstraction ...
> 

Once there is a generic pte_numa handler for example, an arch-specific
overriding of it should raise a red flag for closer inspection.

-- 
Mel Gorman
SUSE Labs

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