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Date: Mon, 14 May 2007 11:12:24 -0500
From: Matt Mackall <mpm@selenic.com>
To: Christoph Lameter <clameter@sgi.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Thomas Graf <tgraf@suug.ch>,
       David Miller <davem@davemloft.net>,
       Andrew Morton <akpm@linux-foundation.org>,
       Daniel Phillips <phillips@google.com>,
       Pekka Enberg <penberg@cs.helsinki.fi>
Subject: Re: [PATCH 0/5] make slab gfp fair
Message-ID: <20070514161224.GC11115@waste.org>
References: <20070514131904.440041502@chello.nl> <Pine.LNX.4.64.0705140852150.10442@schroedinger.engr.sgi.com>
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On Mon, May 14, 2007 at 08:53:21AM -0700, Christoph Lameter wrote:
> On Mon, 14 May 2007, Peter Zijlstra wrote:
> 
> > In the interest of creating a reserve based allocator; we need to make the slab
> > allocator (*sigh*, all three) fair with respect to GFP flags.
> 
> I am not sure what the point of all of this is. 
> 
> > That is, we need to protect memory from being used by easier gfp flags than it
> > was allocated with. If our reserve is placed below GFP_ATOMIC, we do not want a
> > GFP_KERNEL allocation to walk away with it - a scenario that is perfectly
> > possible with the current allocators.
> 
> Why does this have to handled by the slab allocators at all? If you have 
> free pages in the page allocator then the slab allocators will be able to 
> use that reserve.

If I understand this correctly:

privileged thread                      unprivileged greedy process
kmem_cache_alloc(...)
   adds new slab page from lowmem pool
do_io()
                                       kmem_cache_alloc(...)
                                       kmem_cache_alloc(...)
                                       kmem_cache_alloc(...)
                                       kmem_cache_alloc(...)
                                       kmem_cache_alloc(...)
                                       ...
                                          eats it all
kmem_cache_alloc(...) -> ENOMEM
   who ate my donuts?!

But I think this solution is somehow overkill. If we only care about
this issue in the OOM avoidance case, then our rank reduces to a
boolean.

-- 
Mathematics is the supreme nostalgia of our time.