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Message-ID: <CAF1ivSbu4e2MzUB2CPJi92PpkOvLrLA=TO55-=7m07GpYz98Xw@mail.gmail.com>
Date: Wed, 20 May 2015 11:16:20 -0700
From: Ming Lin <mlin@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Ming Lin <mlin@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
Jens Axboe <axboe@...nel.dk>,
Kent Overstreet <kent.overstreet@...il.com>,
Dongsu Park <dpark@...teo.net>,
Christoph Hellwig <hch@...radead.org>,
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,
Lars Ellenberg <drbd-dev@...ts.linbit.com>,
drbd-user@...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: [PATCH v3 01/11] block: make generic_make_request handle
arbitrarily sized bios
On Wed, May 20, 2015 at 5:44 AM, Christoph Hellwig <hch@....de> wrote:
> This needs the patch below for the blk-mq case with multiple queues.
Will add it.
>
> Also I wonder why we really want to push this below ->make_request.
> Moving it just outside ->make_request would make the callchain more
> obvious and avoid the recursion into the driver.
Are you saying move the split to generic_make_request()?
Below is an off-list discussion.
Ming Lei asked:
> > I am wondering why the bio isn't splitted just before q->make_request_fn
> > is called in generic_make_request()? By this way, drivers won't need
> > to call blk_queue_split() at all. Is it because performance reason? or
> > others?
Kent replied:
> > The reasoning was because blk_queue_split() was only an interim solution -
> > there's actually no inherent need to split the bios at all, but not splitting
> > them takes (a small amount of) driver modification. You just have the driver
> > consume the bios incrementally, advancing the iterator and using bi_remaining
> > when they have multiple tags pointing to the same bio.
>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e9bbc04..e7fae76 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1256,6 +1256,8 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
> return;
> }
>
> + blk_queue_split(q, &bio, q->bio_split);
> +
> rq = blk_mq_map_request(q, bio, &data);
> if (unlikely(!rq))
> return;
--
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