linux-kernel - Re: [PATCH 7/8] lguest: trivial guest block driver

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Message-Id: <1171258034.10409.54.camel@localhost.localdomain>
Date:	Mon, 12 Feb 2007 16:27:14 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.osdl.org>
Subject: Re: [PATCH 7/8] lguest: trivial guest block driver

On Mon, 2007-02-12 at 05:43 +0100, Jens Axboe wrote:
> On Mon, Feb 12 2007, Rusty Russell wrote:
> > +	end_request(bd->req, bd->lb_page->result == 1);
>
> You are using the old-style end request handling. So while I generally
> discourage use of end_request(), you seem to have a bigger problem here:

> > +	rq_for_each_bio(bio, req) {
> > +		struct bio_vec *bvec;
> > +		bio_for_each_segment(bvec, bio, idx) {
> > +			BUG_ON(i == LGUEST_MAX_DMA_SECTIONS);
> > +			BUG_ON(!bvec->bv_len);
> > +			dma->addr[i] = page_to_phys(bvec->bv_page)
> > +				+ bvec->bv_offset;
> > +			dma->len[i] = bvec->bv_len;
> > +			len += bvec->bv_len;
> > +			i++;
> > +		}
> > +	}
> > +	if (i < LGUEST_MAX_DMA_SECTIONS)
> > +		dma->len[i] = 0;
> > +	return len;
> > +}
> 
> Here you map the entire request (lets call that segment A..Z), but
> end_request() only completes the first chunk of the request. So
> elv_next_request() will retrieve the same request again, and you'll then
> map B..Z and repeat that transfer. So unless I'm missing some other part
> here (just read it over quickly), you are re-doing large parts of a
> merged request several times.
> 
> So: don't use end_request(). Add some driver helper that does:
> 
> static void lgb_end_request(struct blockdev *bd)
> {
>         int uptodate = bd->lb_page->result == 1;
>         struct request *rq = bd->req;
> 
>         end_that_request_first(rq, uptodate, req->hard_nr_sectors);
>         add_disk_randomness(rq->rq_disk);
>         blkdev_dequeue_request(rq);
>         end_that_request_last(rq, uptodate);
> }
> 
> We could probably even make that a block layer helper, I'm sure others
> could be cleaned up with that as well. You want to use that helper in
> do_lgb_request() as well.

I'm confused.  That code looks like end_request:

void end_request(struct request *req, int uptodate)
{
	if (!end_that_request_first(req, uptodate, req->hard_cur_sectors)) {
		add_disk_randomness(req->rq_disk);
		blkdev_dequeue_request(req);
		end_that_request_last(req, uptodate);
	}
}

Rusty.

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