Date: Tue, 23 Dec 2014 12:41:40 +0100
From: Dongsu Park <dongsu.park@profitbricks.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        Kent Overstreet <kmo@daterainc.com>, Ming Lin <mlin@minggr.net>,
        Al Viro <viro@zeniv.linux.org.uk>, Ming Lei <ming.lei@canonical.com>,
        Neil Brown <neilb@suse.de>, Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>, dm-devel@redhat.com,
        drbd-dev@lists.linbit.com, Jiri Kosina <jkosina@suse.cz>,
        Geoff Levand <geoff@infradead.org>, Jim Paris <jim@jtan.com>,
        Joshua Morris <josh.h.morris@us.ibm.com>,
        Philip Kelleher <pjk1939@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>, Nitin Gupta <ngupta@vflare.org>,
        Oleg Drokin <oleg.drokin@intel.com>,
        Andreas Dilger <andreas.dilger@intel.com>
Subject: Re: [RFC PATCH 01/17] block: make generic_make_request handle
 arbitrarily sized bios
Message-ID: <20141223114140.GB15080@gmail.com>
References: <cover.1419241597.git.dongsu.park@profitbricks.com>
 <d26a9a0fc9a7d1529c115785ee5935d5750782bd.1419241597.git.dongsu.park@profitbricks.com>
 <20141223101614.GA13138@infradead.org>
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Hi Christoph,

On 23.12.2014 02:16, Christoph Hellwig wrote:
> > +void blk_queue_split(struct request_queue *q, struct bio **bio,
> > +		     struct bio_set *bs)
> > +{
> > +	struct bio *split;
> > +
> > +	if ((*bio)->bi_rw & REQ_DISCARD)
> > +		split = blk_bio_discard_split(q, *bio, bs);
> > +	else if ((*bio)->bi_rw & REQ_WRITE_SAME)
> > +		split = blk_bio_write_same_split(q, *bio, bs);
> > +	else
> > +		split = blk_bio_segment_split(q, *bio, q->bio_split);
> > +
> > +	if (split) {
> > +		bio_chain(split, *bio);
> > +		generic_make_request(*bio);
> > +		*bio = split;
> > +	}
> > +}
> > +EXPORT_SYMBOL(blk_queue_split);
> 
> I think blk_queue_split needs to explicitly skip BLOCK_PC bios.  Those
> are SCSI pass through ioctls that we can't split due to their opaque
> nature.

You mean, checking rq->cmd_type == REQ_TYPE_BLOCK_PC, right?

I'm wondering about how to check that in blk_queue_split().
At the moment when blk_queue_split() is called, it's even before a request
is mapped e.g. in blk_sq_make_request().
Unlike scsi drivers where it's easy to get cmd->rq, blk_queue_split()
doesn't seem to be able to get a request by blk_get_request().

Or am I missing something?

Thanks,
Dongsu
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