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Date: Mon, 12 Jul 2010 15:22:54 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Dave Chinner <david@fromorbit.com>, Christoph Hellwig <hch@infradead.org>,
        Martin Bligh <mbligh@google.com>, Michael Rubin <mrubin@google.com>,
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        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases
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On Mon, 12 Jul 2010 23:52:39 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> > Also, I'd prefer that the
> > comments remain somewhat more descriptive of the circumstances that
> > we are operating under. Comments like "retry later to avoid blocking
> > writeback of other inodes" is far, far better than "retry later"
> > because it has "why" component that explains the reason for the
> > logic. You may remember why, but I sure won't in a few months time....

me2 (of course).  This code is waaaay too complex to be scrimping on comments.

> Ah yes the comment is too simple. However the redirty_tail() is not to
> avoid blocking writeback of other inodes, but to avoid eating 100% CPU
> on busy retrying a dirty inode/page that cannot perform writeback for
> a while. (In theory redirty_tail() can still busy retry though, when
> there is only one single dirty inode.) So how about
> 
>         /*
>          * somehow blocked: avoid busy retrying
>          */

That's much too short.  Expand on the "somehow" - provide an example,
describe the common/expected cause.  Fully explain what the "busy"
retry _is_ and how it can come about.