Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC_ALL writebacks

From: Jens Axboe <jens.axboe@oracle.com>
To: Theodore Tso <tytso@mit.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Developers List <linux-kernel@vger.kernel.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	jack@suse.cz
Subject: Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC_ALL writebacks
Date: Wed, 8 Apr 2009 07:58:54 +0200	[thread overview]
Message-ID: <20090408055853.GJ5178@kernel.dk> (raw)
In-Reply-To: <20090407214421.GA7031@mit.edu>

On Tue, Apr 07 2009, Theodore Tso wrote:
> On Tue, Apr 07, 2009 at 09:32:39PM +0200, Jens Axboe wrote:
> > 
> > This should be all you need, forget the below. The manual unplugging
> > only really makes sense for very select situations, since it'll be done
> > when someone waits on the buffer anyway.
> 
> Thanks, ok; I missed the "sync_buffer" in the call to wait_on_bit() in
> __wait_on_buffer().  It also wasn't obvious to me that
> wait_on_writeback_range() ultimately ends up calling aops->sync_page
> which for filemap.c will call blk_run_backing_dev() --- and that
> blk_run_*() results in the right device queue getting unplugged.  So
> this would be a nice thing to get documented in
> Documentation/biodoc.txt.

I'm not even sure what is documented about this sort of thing in biodoc,
but it's actually somewhat outside the realm of block layer
documentation. The block layer should document that you need to call
unplug if you are going to be waiting for that IO. How the vm code does
it is implementation detail mostly, the important thing is that it gets
done :-)

-- 
Jens Axboe