linux-kernel - Re: [PATCH 6/7] writeback: separate starting of sync vs opportunistic writeback

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Message-ID: <20090915091402.GG23126@kernel.dk>
Date:	Tue, 15 Sep 2009 11:14:02 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, tytso@....edu, akpm@...ux-foundation.org,
	trond.myklebust@....uio.no
Subject: Re: [PATCH 6/7] writeback: separate starting of sync vs
	opportunistic writeback

On Tue, Sep 15 2009, Jan Kara wrote:
> On Mon 14-09-09 21:42:43, Jens Axboe wrote:
> > On Mon, Sep 14 2009, Jens Axboe wrote:
> > > On Mon, Sep 14 2009, Christoph Hellwig wrote:
> > > > On Mon, Sep 14, 2009 at 03:33:07PM +0200, Jan Kara wrote:
> > > > > On Mon 14-09-09 11:36:33, Jens Axboe wrote:
> > > > > > bdi_start_writeback() is currently split into two paths, one for
> > > > > > WB_SYNC_NONE and one for WB_SYNC_ALL. Add bdi_sync_writeback()
> > > > > > for WB_SYNC_ALL writeback and let bdi_start_writeback() handle
> > > > > > only WB_SYNC_NONE.
> > > > >   What I don't like about this patch is that if somebody sets up
> > > > > writeback_control with WB_SYNC_ALL mode set and then submits it to disk via
> > > > > bdi_start_writeback() it will just silently convert his writeback to an
> > > > > asynchronous one.
> > > > >   So I'd maybe leave setting of sync_mode to the caller and just WARN_ON if
> > > > > it does not match the purpose of the function...
> > > > 
> > > > Or initialize the wb entirely inside these functions.  For the sync case
> > > > we really only need a superblock as argument, and for writeback it's
> > > > bdi + nr_pages.  And also make sure they consistenly return void as
> > > > no one cares about the return value.
> > > 
> > > Yes, I thought about doing that and like that better than the warning.
> > > Just pass in the needed args and allocate+fill the wbc on stack. I'll
> > > make that change.
> > 
> > That works out much better, imho:
> > 
> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=270c12655d7d11e234d335a8ab0540c02c034b66
>   Yeah, the code looks better. BTW, how about converting also
> bdi_writeback_all() to get superblock and nr_pages as an argument?
> Currently it seems to be the only place "above" flusher thread which uses
> wbc and it's just constructed in the callers of bdi_writeback_all() and
> then disassembled inside the function...

Yes good point, I'll include that too. Thanks!

-- 
Jens Axboe

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