linux-kernel - Re: [RFC PATCH 01/17] block: make generic_make

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Date:	Tue, 23 Dec 2014 12:41:40 +0100
From:	Dongsu Park <dongsu.park@...fitbricks.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kmo@...erainc.com>,
	Ming Lin <mlin@...ggr.net>, Al Viro <viro@...iv.linux.org.uk>,
	Ming Lei <ming.lei@...onical.com>, Neil Brown <neilb@...e.de>,
	Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
	drbd-dev@...ts.linbit.com, Jiri Kosina <jkosina@...e.cz>,
	Geoff Levand <geoff@...radead.org>, Jim Paris <jim@...n.com>,
	Joshua Morris <josh.h.morris@...ibm.com>,
	Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>
Subject: Re: [RFC PATCH 01/17] block: make generic_make_request handle
 arbitrarily sized bios

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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