linux-kernel - Re: [mm][RFC][PATCH 0/11] mm accessor updates.

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Date:	Thu, 17 Dec 2009 00:01:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...e.hu" <mingo@...e.hu>, minchan.kim@...il.com
Subject: Re: [mm][RFC][PATCH 0/11] mm accessor updates.

On Wed, 2009-12-16 at 10:27 -0600, Christoph Lameter wrote:
> On Wed, 16 Dec 2009, Andi Kleen wrote:
> 
> > > Do you have alternative recommendation rather than wrapping all accesses by
> > > special functions ?
> >
> > Work out what changes need to be done for ranged mmap locks and do them all
> > in one pass.
> 
> Locking ranges is already possible through the split ptlock and
> could be enhanced through placing locks in the vma structures.
> 
> That does nothing solve the basic locking issues of mmap_sem. We need
> Kame-sans abstraction layer. A vma based lock or a ptlock still needs to
> ensure that the mm struct does not vanish while the lock is held.

It should, you shouldn't be able to remove a mm while there's still
vma's around, and you shouldn't be able to remove a vma when there's
still pagetables around. And if you rcu-free all of them you're stable
enough for lots of speculative behaviour.

No need to retain mmap_sem for any of that.

As for per-vma locks, those are pretty much useless too, there's plenty
applications doing lots of work on a few very large vmas.

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