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

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Message-ID: <20090915140145.GG12169@duck.suse.cz>
Date:	Tue, 15 Sep 2009 16:01:45 +0200
From:	Jan Kara <jack@...e.cz>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>, Jan Kara <jack@...e.cz>,
	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 15-09-09 09:08:29, Christoph Hellwig wrote:
> On Tue, Sep 15, 2009 at 03:04:19PM +0200, Jens Axboe wrote:
> > 
> > >   Let's have a look at the flags in wbc:
> > >   nonblocking - Currently only set by direct callers of ->writepage() BUT
> > >                 originally wb_kupdate() and background_writeout() also
> > >                 set this flag. Since filesystems and write_cache_pages()
> > > 		use the flag we should set it for equivalent writeouts as
> > >                 well. This should be fixed...
> > 
> > Since this is all handled by the dedicated thread now, dropping the
> > nonblocking bit was on purpose. What would the point be, except for
> > stopping pdflush being blocked on request allocation?
> 
> Note that this flag just caused utter mess traditionally.  btrfs decided
> to ignore it completely and ext4 partially.  Removing this check in
> XFS increases large bufferd write loads massively.
> 
> Just half-removing it is a bad idea, though - if you don't set it
> anymore please kill it entirely.
  The nonblocking flag is still set for writeback done for memory reclaim.
OTOH the only real consumer of this flag now seems to be
__block_write_full_page() which does trylock_buffer() in case of
nonblocking writeback. I'm undecided whether it makes sence or not.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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