linux-kernel - Re: [PATCH 12/13] writeback: try more writeback as long as something was written

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Message-ID: <20100805170016.GE3535@quack.suse.cz>
Date:	Thu, 5 Aug 2010 19:00:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 12/13] writeback: try more writeback as long as
 something was written

On Fri 06-08-10 00:11:03, Wu Fengguang wrote:
> writeback_inodes_wb()/__writeback_inodes_sb() are not aggressive in that
> they only populate b_io when necessary at entrance time. When the queued
> set of inodes are all synced, they just return, possibly with
> wbc.nr_to_write > 0.
> 
> For kupdate and background writeback, there may be more eligible inodes
> sitting in b_dirty when the current set of b_io inodes are completed. So
> it is necessary to try another round of writeback as long as we made some
> progress in this round. When there are no more eligible inodes, no more
> inodes will be enqueued in queue_io(), hence nothing could/will be
> synced and we may safely bail.
  This looks like a sane thing to do. Just one comment below...
 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  fs/fs-writeback.c |   19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-08-05 23:30:27.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-08-05 23:30:45.000000000 +0800
> @@ -654,20 +654,23 @@ static long wb_writeback(struct bdi_writ
>  		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
>  
>  		/*
> -		 * If we consumed everything, see if we have more
> +		 * Did we write something? Try for more
> +		 *
> +		 * This is needed _before_ the b_more_io test because the
> +		 * background writeback moves inodes to b_io and works on
  Well, this applies generally to any writeback, not just a background one
right? Whenever we process all inodes from b_io list and move them
somewhere else than b_more_io, then this applies. Some new dirty data could
have arrived while we were doing the write... I'm just afraid that in some
pathological cases this could result in bad writeback pattern - like if
there is some process which manages to dirty just a few pages while we are
doing writeout, this looping could result in writing just a few pages in
each round which is bad for fragmentation etc.
  Actually, this comment probably also applies to your patch where you
change the queueing logic in writeback_single_inode(), doesn't it?

								Honza

> +		 * them in batches (in order to sync old pages first).  The
> +		 * completion of the current batch does not necessarily mean
> +		 * the overall work is done.
>  		 */
> -		if (wbc.nr_to_write <= 0)
> +		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
>  			continue;
> +
>  		/*
> -		 * Didn't write everything and we don't have more IO, bail
> +		 * Nothing written and no more inodes for IO, bail
>  		 */
>  		if (list_empty(&wb->b_more_io))
>  			break;
> -		/*
> -		 * Did we write something? Try for more
> -		 */
> -		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
> -			continue;
> +
>  		/*
>  		 * Nothing written. Wait for some inode to
>  		 * become available for writeback. Otherwise
> 
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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