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Date: Mon, 15 Mar 2010 13:09:35 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>, linux-mm@kvack.org,
       Nick Piggin <npiggin@suse.de>, Chris Mason <chris.mason@oracle.com>,
       Jens Axboe <jens.axboe@oracle.com>, linux-kernel@vger.kernel.org,
       gregkh@novell.com
Subject: Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone
 pressure
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On Mon, 15 Mar 2010 13:34:50 +0100
Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> wrote:

> c) If direct reclaim did reasonable progress in try_to_free but did not
> get a page, AND there is no write in flight at all then let it try again
> to free up something.
> This could be extended by some kind of max retry to avoid some weird
> looping cases as well.
> 
> d) Another way might be as easy as letting congestion_wait return
> immediately if there are no outstanding writes - this would keep the 
> behavior for cases with write and avoid the "running always in full 
> timeout" issue without writes.

They're pretty much equivalent and would work.  But there are two
things I still don't understand:

1: Why is direct reclaim calling congestion_wait() at all?  If no
writes are going on there's lots of clean pagecache around so reclaim
should trivially succeed.  What's preventing it from doing so?

2: This is, I think, new behaviour.  A regression.  What caused it?