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Date: Tue, 23 Dec 2014 02:16:14 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Dongsu Park <dongsu.park@profitbricks.com>
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Cc: Jens Axboe <axboe@kernel.dk>, Oleg Drokin <oleg.drokin@intel.com>,
	Nitin Gupta <ngupta@vflare.org>,
	Mike Snitzer <snitzer@redhat.com>, Ming Lin <mlin@minggr.net>,
	Minchan Kim <minchan@kernel.org>, Neil Brown <neilb@suse.de>,
	Jiri Kosina <jkosina@suse.cz>, Ming Lei <ming.lei@canonical.com>,
	linux-kernel@vger.kernel.org, Jim Paris <jim@jtan.com>,
	Philip Kelleher <pjk1939@linux.vnet.ibm.com>,
	Christoph Hellwig <hch@infradead.org>, Geoff Levand <geoff@infradead.org>,
	Kent Overstreet <kmo@daterainc.com>, Al Viro <viro@zeniv.linux.org.uk>,
	Andreas Dilger <andreas.dilger@intel.com>, dm-devel@redhat.com,
	Joshua Morris <josh.h.morris@us.ibm.com>,
	Alasdair Kergon <agk@redhat.com>, drbd-dev@lists.linbit.com
Subject: Re: [Drbd-dev] [RFC PATCH 01/17] block: make generic_make_request
 handle arbitrarily sized bios
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> +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.