linux-kernel - Re: [PATCH 3/5] writeback: prevent sync livelock with the sync

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Date:	Fri, 30 Jul 2010 13:17:46 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 3/5] writeback: prevent sync livelock with the
 sync_after timestamp

On Thu, Jul 29, 2010 at 11:02:41PM +0800, Jan Kara wrote:
>   Hi Fengguang,
> 
> On Thu 29-07-10 19:51:45, Wu Fengguang wrote:
> > The start time in writeback_inodes_wb() is not very useful because it
> > slips at each invocation time. Preferrably one _constant_ time shall be
> > used at the beginning to cover the whole sync() work.
> > 
> > The newly dirtied inodes are now guarded at the queue_io() time instead
> > of the b_io walk time. This is more natural: non-empty b_io/b_more_io
> > means "more work pending".
> > 
> > The timestamp is now grabbed the sync work submission time, and may be
> > further optimized to the initial sync() call time.
>   The patch seems to have some issues...
> 
> > +	if (wbc->for_sync) {
>   For example this is never set. You only set wb->for_sync.

Ah right.

> > +		expire_interval = 1;
> > +		older_than_this = wbc->sync_after;
>   And sync_after is never set either???

Sorry I must lose some chunk when rebasing the patch ..

> > -	if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
> > +	if (list_empty(&wb->b_io))
> >  		queue_io(wb, wbc);
>   And what is the purpose of this? It looks as an unrelated change to me.

Yes it's not tightly related. It may be simpler to do

 -	if (!wbc->for_kupdate || list_empty(&wb->b_io))
 +	if (list_empty(&wb->b_io))

in the previous patch "writeback: sync expired inodes first in
background writeback".

Thanks,
Fengguang
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