linux-kernel - Re: [RFC] [PATCH] A clean aEvgeniy pproach to writeout throttling

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Message-ID: <20071210121627.GF5008@kernel.dk>
Date:	Mon, 10 Dec 2007 13:16:27 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC] [PATCH] A clean aEvgeniy pproach to writeout throttling

On Mon, Dec 10 2007, Daniel Phillips wrote:
> On Monday 10 December 2007 03:41, Jens Axboe wrote:
> > On Mon, Dec 10 2007, Daniel Phillips wrote:
> > > +       if (q && q->metric && !bio->bi_queue) {
> > >
> > > This prevents any reference to bi_bdev after the intial call to
> > > generic_make_request.  Thanks to Evgeniy for pointing out the need
> > > for this measure on the last go round.
> >
> > Which saves the initial target, for ease of accounting at end io time
> > - that's not the point. What happens when ->make_request_fn() changes
> > bio->bi_bdev and returns 1, causing another iteration of the
> > __generic_make_request() loop? 'q' is no longer the valid target,
> > bdev_get_queue(bio->bi_bdev) is.
> 
> What happens on the second iteration of a recursive submission loop is 
> exactly nothing, as is right and proper.  The throttling has already 
> been done, and all the state necessary to perform the unthrottle was 
> recorded in the bio.  Everything seems to be in order there, and the 
> algorithm does indeed perform its function as designed, though to be 
> sure we have not tested it on -mm branch, only on mainline.

OK, let me get the neon out then. This has nothing to do with
throttling, I thought I made it clear that I get why you store the
origin queue in ->bi_queue. I'm concerned with the workings of
redirecting a bio. Previously we looked up the queue associated with
bio->bi_bdev inside the loop in __generic_make_request(), as is REQUIRED
to correctly locate a DIFFERENT queue if bio->bi_bdev has been changed
to point somewhere else.

Clear?

-- 
Jens Axboe

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