linux-kernel - Re: [PATCH 03/17] writeback: introduce writeback_control.inodes

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Date:	Fri, 13 May 2011 11:36:05 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mel@....ul.ie>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/17] writeback: introduce
 writeback_control.inodes_cleaned

On Fri, May 13, 2011 at 06:44:20AM +0800, Dave Chinner wrote:
> On Thu, May 12, 2011 at 09:57:09PM +0800, Wu Fengguang wrote:
> > The flusher works on dirty inodes in batches, and may quit prematurely
> > if the batch of inodes happen to be metadata-only dirtied: in this case
> > wbc->nr_to_write won't be decreased at all, which stands for "no pages
> > written" but also mis-interpreted as "no progress".
> > 
> > So introduce writeback_control.inodes_cleaned to count the inodes get
> > cleaned.  A non-zero value means there are some progress on writeback,
> > in which case more writeback can be tried.
> 
> Why introduce a new field for this?

Yeah sorry, but this is an intermediate field that will be removed in
patch 14.

> Just decrement nr_to_write for every write_inode() call made in
> writeback_single_inode()....

There are two problems

- nr_to_write has always been "# of pages written" and writeback_sb_inodes()
  is actually making use of it to do page accounting in work->nr_pages.

- write_inode() does not always succeed, and its return value is not
  reliable on every filesystem.. (I actually tried this approach in v1
  and found sync(1) hang on NFS)

Thanks,
Fengguang
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