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Subject: Re: [patch 1/7] cpusets: add dirty map to struct address_space
From: Peter Zijlstra <peterz@infradead.org>
To: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>, Paul Menage <menage@google.com>,
       Derek Fults <dfults@sgi.com>, linux-kernel@vger.kernel.org
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Date: Tue, 28 Oct 2008 18:37:39 +0100
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On Tue, 2008-10-28 at 09:08 -0700, David Rientjes wrote:

> This patch implements the management of dirty node maps for an address
> space through the following functions:
> 
> cpuset_clear_dirty_nodes(mapping)	Clear the map of dirty nodes
> 
> cpuset_update_nodes(mapping, page)	Record a node in the dirty nodes
> 					map
> 
> cpuset_init_dirty_nodes(mapping)	Initialization of the map
> 
> 
> The dirty map may be stored either directly in the mapping (for NUMA
> systems with less then BITS_PER_LONG nodes) or separately allocated for
> systems with a large number of nodes (f.e. ia64 with 1024 nodes).
> 
> Updating the dirty map may involve allocating it first for large
> configurations.  Therefore, we protect the allocation and setting of a
> node in the map through the tree_lock.  The tree_lock is already taken
> when a page is dirtied so there is no additional locking overhead if we
> insert the updating of the nodemask there.

I find this usage of tree lock most bothersome, as my concurrent
pagecache patches take the lock out. In which case this _does_ cause
extra locking overhead.