From: Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge
Date: Tue, 25 Jan 2011 15:46:06 -0500
Message-ID: <20110125204606.GE10664@redhat.com>
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Cc: Mike Snitzer <snitzer@redhat.com>, axboe@kernel.dk, tytso@mit.edu,
	djwong@us.ibm.com, shli@kernel.org, neilb@suse.de,
	adilger.kernel@dilger.ca, jack@suse.cz,
	linux-kernel@vger.kernel.org, kmannth@us.ibm.com, cmm@us.ibm.com,
	linux-ext4@vger.kernel.org, rwheeler@redhat.com, hch@lst.de,
	josef@redhat.com
To: Tejun Heo <tj@kernel.org>
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Sender: linux-ext4-owner@vger.kernel.org

On Sun, Jan 23, 2011 at 11:31:33AM +0100, Tejun Heo wrote:
> On Fri, Jan 21, 2011 at 07:49:55PM -0500, Mike Snitzer wrote:
> > > + * If the device doesn't have writeback cache, FLUSH and FUA don't make any
> > > + * difference.  The requests are either completed immediately if there's no
> > > + * data or executed as normal requests otherwise.
> > 
> > For devices without a writeback cache, I'm not seeing where pure flushes
> > are completed immediately.  But I do see where data is processed
> > directly in blk_insert_flush().
> 
> Yeah, it does.  Pure flushes on a device w/o writeback cache, @policy
> is zero and blk_flush_complete_seq() will directly proceed to
> REQ_FSEQ_DONE.

I see following code in __generic_make_request(). I am wondering if empty
flushes will be completed here itself if device does not have writeback
cache.

                /*
                 * Filter flush bio's early so that make_request based
                 * drivers without flush support don't have to worry
                 * about them.
                 */
                if ((bio->bi_rw & (REQ_FLUSH | REQ_FUA)) && !q->flush_flags) {
                        bio->bi_rw &= ~(REQ_FLUSH | REQ_FUA);
                        if (!nr_sectors) {
                                err = 0;
                                goto end_io;
                        }
                }

Thanks
Vivek