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Date: Sat, 11 Apr 2009 11:49:20 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>, linux-kernel@vger.kernel.org,
       lpk@581.spb.su, wli@movementarian.org, torvalds@linux-foundation.org,
       yinghan@google.com, Nick Piggin <npiggin@suse.de>,
       Hugh Dickins <hugh@veritas.com>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 6/9] readahead: clean up and simplify the code for
 filemap  page fault readahead
Message-Id: <20090411114920.b9d2493c.akpm@linux-foundation.org>
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On Sat, 11 Apr 2009 22:58:31 +0900 KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > Ah. __But I have a note here that I didn't like it, because it adds lots
> > of new spinlocking to fastpaths. __So I'll leave things as they stand
> > until we have had a little talk about that.
> 
> add?
> 
> old code: grab zone->lru_lock via mark_page_accessed()
> new code: grab zone->lru_lock via update_reclaim_stat()
> 
> one remove, one add.
> 

mm-update_page_reclaim_stat-is-called-from-page-fault-path.patch adds
new calls to update_page_reclaim_stat() into do_swap_page() and
filemap_fault().  update_page_reclaim_stat() does spin_lock_irq() and
spin_unlock_irq().  It looks like a net slowdown to me.

> But I agree its lock can be removed maybe..

It would be nice to try to do something about it - every little bit
counts.