linux-kernel - Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC

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Date:	Fri, 11 Sep 2009 20:14:32 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, tytso@....edu, akpm@...ux-foundation.org,
	jack@...e.cz, trond.myklebust@....uio.no
Subject: Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for
	WB_SYNC_NONE writeout

On Fri, Sep 11 2009, Christoph Hellwig wrote:
> > -static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> > +static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> > +				 struct writeback_control *wbc)
> >  {
> >  	struct bdi_work *work;
> >  
> > @@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> >  	if (work)
> >  		bdi_work_init(work, wbc);
> >  
> > -	return work;
> > +	bdi_queue_work(bdi, work);
> 
> This is now the only caller of bdi_queue_work that has a NULL work
> argument.  I would recommend removing the !work half of bdi_queue_work
> and just inline it into this function (or make it a separate helper).

Yep agreed.

> >  /*
> > + * Schedule writeback for all backing devices. Can only be used for
> > + * WB_SYNC_NONE writeback, WB_SYNC_ALL should use bdi_start_writeback()
> > + * and pass in the superblock.
> >   */
> >  static void bdi_writeback_all(struct writeback_control *wbc)
> >  {
> >  	struct backing_dev_info *bdi;
> >  
> > +	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
> > +
> >  	spin_lock(&bdi_lock);
> >  
> >  	list_for_each_entry(bdi, &bdi_list, bdi_list) {
> >  		if (!bdi_has_dirty_io(bdi))
> >  			continue;
> >  
> > +		bdi_alloc_queue_work(bdi, wbc);
> >  	}
> 
> 
> Much nicer than before.  Could be even further simplified to:
> 
> 		if (bdi_has_dirty_io(bdi))
> 			bdi_alloc_queue_work(bdi, wbc);

Sure, matter of style.

> > @@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  {
> >  	struct writeback_control wbc = {
> >  		.sb		= sb,
> > +		.bdi		= sb->s_bdi,
> >  		.sync_mode	= WB_SYNC_ALL,
> >  		.range_start	= 0,
> >  		.range_end	= LLONG_MAX,
> > @@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  	long nr_to_write = LONG_MAX; /* doesn't actually matter */
> >  
> >  	wbc.nr_to_write = nr_to_write;
> > -	bdi_writeback_all(&wbc);
> > +	bdi_start_writeback(&wbc);
> 
> So here we have a WB_SYNC_ALL caller of bdi_writeback_all and the
> only other caller is WB_SYNC_NONE.  Given that after patch two those
> are entirely different codepathes in bdi_start_writeback I would just
> split bdi_start_writeback into two separate functions.

Sure, it'll draw a sharper line between WB_SYNC_NONE and WB_SYNC_ALL.
I'll work on this and the bdi cap dirty flag removal as a follow up.

-- 
Jens Axboe

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