From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: [patch 1/2]block: handle merged discard request
Date: Mon, 20 Aug 2012 10:12:29 -0400
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To: Christoph Hellwig <hch@infradead.org>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>, Shaohua Li <shli@kernel.org>, Vivek Goyal <vgoyal@redhat.com>, axboe@kernel.dk, linux-kernel@vger.kernel.org, neilb@suse.de, linux-scsi@vger.kernel.org
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On Mon, Aug 20 2012 at  9:58am -0400,
Christoph Hellwig <hch@infradead.org> wrote:

> On Mon, Aug 20, 2012 at 09:57:39AM -0400, Mike Snitzer wrote:
> > But I haven't put my finger on _why_ a discard bio has bio->bi_io_vec
> > (but given my use of DM, bio comes from bio_alloc_bioset, and DM passes
> > original bio->bi_max_vecs for nr_iovecs).
> 
> TRIM has a payload and we cheay by preallocation a data page for it.

Thought we pushed that down?  Hence sd_setup_discard_cmnd's
alloc_page + blk_add_request_payload hack.