Date:	Wed, 20 Apr 2011 10:02:59 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>, Matthew Wilcox <matthew@....cx>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	linux-parisc@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, x86 maintainers <x86@...nel.org>,
	linux-arch@...r.kernel.org, Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards

On Wed, 2011-04-20 at 09:50 -0500, Christoph Lameter wrote:
> On Wed, 20 Apr 2011, James Bottomley wrote:
> 
> >      1. We can look at what imposing NUMA on the DISCONTIGMEM archs
> >         would do ... the embedded ones are going to be hardest hit, but
> >         if it's not too much extra code, it might be palatable.
> >      2. The other is that we can audit mm to look at all the node
> >         assumptions in the non-numa case.  My suspicion is that
> >         accidentally or otherwise, it mostly works for the normal case,
> >         so there might not be much needed to pull it back to working
> >         properly for DISCONTIGMEM.
> 
> The older code may work. SLAB f.e. does not call page_to_nid() in the
> !NUMA case but keeps special metadata structures around in each slab page
> that records the node used for allocation. The problem is with new code
> added/revised in the last 5 years or so that uses page_to_nid() and
> allocates only a single structure for !NUMA. There are also VM_BUG_ONs in
> the page allocator that should trigger if page_to_nid() returns strange
> values. I wonder why that never occurred.

Actually, I think slab got changed when discontigmem was added ...
that's why it all works OK.

> >      3. Finally we could look at deprecating DISCONTIGMEM in favour
> of >         SPARSEMEM, but we'd still need to fix -stable for that case.
> >         Especially as it will take time to convert all the architectures
> 
> The fix needed is to mark DISCONTIGMEM without NUMA as broken for now. We
> need an audit of the core VM before removing that or making it contingent
> on the configurations of various VM subsystems.

Don't be stupid ... that would cause six architectures to get marked
broken.

> > I'm certainly with Matthew: DISCONTIGMEM is supposed to be a lightweight
> > framework which allows machines with split physical memory ranges to
> > work.  That's a very common case nowadays.  Numa is supposed to be a
> > heavyweight framework to preserve node locality for non-uniform memory
> > access boxes (which none of the DISCONTIGMEM && !NUMA systems are).
> 
> Well yes but we have SPARSE for that today. DISCONTIG with multiple per
> pgdat structures in a !NUMA case is just weird and unexpected for many who
> have done VM coding in the last years.

Look, I'm not really interested in who understands what.  The fact is we
have six architectures with the possibility for DISCONTIGMEM && !NUMA,
so that's the case we need to fix in -stable.

They oops with SLUB, as far as I can tell, there are still no oops
reports with SLAB.  The simplest -stable fix seems to be to mark SLUB
broken on DISCONTIGMEM && !NUMA.

James


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