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From: Jens Axboe <jens.axboe@oracle.com>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
	virtualization <virtualization@lists.osdl.org>
Subject: Re: [PATCH 7/8] lguest: trivial guest block driver
Date: Mon, 12 Feb 2007 06:33:56 +0100	[thread overview]
Message-ID: <20070212053356.GC3999@kernel.dk> (raw)
In-Reply-To: <20070212053204.GB3999@kernel.dk>

On Mon, Feb 12 2007, Jens Axboe wrote:
> On Mon, Feb 12 2007, Rusty Russell wrote:
> > 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);
> > 	}
> > }
> 
> Note hard_cur_sectors vs hard_nr_sectors. The former refers to the first
> segment sector count, the latter to the total sector count in the
> request. Hence the difference!

Also, that is why my example code doesn't check the
end_that_request_first() return value, it must be 0 or it would be
buggy. Alternatively,

        if (end_that_request_first(rq, uptodate, rq->hard_nr_sectors))
                BUG()

would make that more clear. Or just a nice little comment. Anyway, if
you fixup lguest I'll add a kernel helper that we can switch it to once
merged.

-- 
Jens Axboe


  reply	other threads:[~2007-02-12  5:33 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-12  3:32 [PATCH 1/7] cleanup: paravirt unhandled fallthrough Rusty Russell
2007-02-12  3:33 ` [PATCH 2/7] cleanup: Initialize esp0 properly all the time Rusty Russell
2007-02-12  3:34   ` [PATCH 3/7] cleanup: Make hvc_console.c compile on non-PowerPC Rusty Russell
2007-02-12  3:35     ` [PATCH 4/7] cleanup: Move mce_disabled to asm/mce.h Rusty Russell
2007-02-12  3:36       ` [PATCH 5/7] cleanup: Rename cpu_gdt_descr and remove extern declaration from smpboot.c Rusty Russell
2007-02-12  3:37         ` [PATCH 6/7] cleanup: Remove extern declaration from mm/discontig.c, put in header Rusty Russell
2007-02-12  3:39           ` [PATCH 7/7] cleanup: make disable_acpi() valid w/o CONFIG_ACPI Rusty Russell
2007-02-12  3:41             ` [PATCH 1/2] lguest preparation: EXPORT_SYMBOL_GPL 5 functions Rusty Russell
2007-02-12  3:42               ` [PATCH 2/2] lguest preparation: expose futex infrastructure: get_futex_key, get_key_refs and drop_key_refs Rusty Russell
2007-02-12  3:44                 ` [PATCH 1/8] lguest: Kconfig and headers Rusty Russell
2007-02-12  3:46                   ` [PATCH 2/8] lguest: the host code (lg.ko) Rusty Russell
2007-02-12  3:48                     ` [PATCH 3/8] lguest: Guest code Rusty Russell
2007-02-12  3:50                       ` [PATCH 4/8] lguest: Makefile Rusty Russell
2007-02-12  3:52                         ` [PATCH 5/8] lguest: trivial guest network driver Rusty Russell
2007-02-12  3:53                           ` [PATCH 6/8] lguest: trivial guest console driver Rusty Russell
2007-02-12  3:54                             ` [PATCH 7/8] lguest: trivial guest block driver Rusty Russell
2007-02-12  3:55                               ` [PATCH 8/8] lguest: documentatation and example launcher Rusty Russell
2007-02-12  4:43                               ` [PATCH 7/8] lguest: trivial guest block driver Jens Axboe
2007-02-12  5:27                                 ` Rusty Russell
2007-02-12  5:32                                   ` Jens Axboe
2007-02-12  5:33                                     ` Jens Axboe [this message]
2007-02-12  7:09                                     ` Rusty Russell
2007-02-12  7:09                                       ` Rusty Russell
2007-02-12 15:01                                       ` Jens Axboe
2007-02-13  0:25                                         ` Rusty Russell
2007-02-13  0:25                                           ` Rusty Russell
2007-02-13  0:44                                           ` Jens Axboe
2007-02-12 15:55                           ` [PATCH 5/8] lguest: trivial guest network driver Herbert Xu
2007-02-13  2:15                             ` Rusty Russell
2007-02-13 14:06                               ` Herbert Xu
2007-02-14  4:47                                 ` Rusty Russell
2007-02-14 13:57                                   ` Herbert Xu
2007-02-14 23:00                                     ` Rusty Russell
2007-02-12 16:02                   ` [PATCH 1/8] lguest: Kconfig and headers James Morris
2007-02-13  5:09             ` [PATCH 7/7] cleanup: make disable_acpi() valid w/o CONFIG_ACPI Len Brown
2007-02-12  9:16         ` [PATCH 5/7] cleanup: Rename cpu_gdt_descr and remove extern declaration from smpboot.c Zachary Amsden

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