linux-kernel - Re: [patch 7/7] powerpc: lockless get_user_pages

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Date:	Wed, 11 Jun 2008 06:47:16 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org, paulus@...ba.org
Subject: Re: [patch 7/7] powerpc: lockless get_user_pages_fast

On Tue, Jun 10, 2008 at 09:40:25PM -0700, Christoph Lameter wrote:
> On Wed, 11 Jun 2008, Nick Piggin wrote:
> 
> > > This is reversing the modification to make get_page_unless_zero() usable 
> > > with compound page heads. Will break the slab defrag patchset.
> > 
> > Is the slab defrag patchset in -mm? Because you ignored my comment about
> > this change that assertions should not be weakened until required by the
> > actual patchset. I wanted to have these assertions be as strong as
> > possible for the lockless pagecache patchset.
> 
> So you are worried about accidentally using get_page_unless_zero on a 
> compound page? What would be wrong about that?

Unexpected. Compound pages should have no such races that require
get_page_unless_zero that we very carefully use in page reclaim.

If you don't actually know whether you have a reference to the
thing or not before trying to operate on it, then you're almost
definitely got refcount wrong. How does slab defrag use it?

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