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Subject: Re: [PATCH 00/10] foundations for reserve-based allocation
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Daniel Phillips <phillips@phunq.net>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       David Miller <davem@davemloft.net>,
       Andrew Morton <akpm@linux-foundation.org>,
       Daniel Phillips <phillips@google.com>,
       Pekka Enberg <penberg@cs.helsinki.fi>,
       Christoph Lameter <clameter@sgi.com>, Matt Mackall <mpm@selenic.com>,
       Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
       Steve Dickson <SteveD@redhat.com>
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Date: Mon, 06 Aug 2007 21:36:58 +0200
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On Mon, 2007-08-06 at 12:31 -0700, Daniel Phillips wrote:
> On Monday 06 August 2007 11:17, Peter Zijlstra wrote:
> > And how do we know a page was taken out of the reserves?
> 
> Why not return that in the low bit of the page address?  This is a 
> little more cache efficient, does not leave that odd footprint in the 
> page union and forces the caller to examine the 
> alloc_pages(...P_MEMALLOC) return, making it harder to overlook the 
> fact that it got a page out of reserve and forget to put one back 
> later.

This would require auditing all page allocation sites to ensure they
ever happen under PF_MEMALLOC or the like. Because if an allocator ever
fails to check the low bit and assumes its a valid struct page *, stuff
will go *bang*.