linux-kernel - Re: [PATCH 6/6] writeback: merge for_kupdate and !for

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Date:	Mon, 12 Jul 2010 15:22:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Martin Bligh <mbligh@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases

On Mon, 12 Jul 2010 23:52:39 +0800
Wu Fengguang <fengguang.wu@...el.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.

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