Date: Thu, 21 Dec 2006 19:42:04 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: Mike Christie <michaelc@cs.wisc.edu>
Cc: device-mapper development <dm-devel@redhat.com>, mchristi@redhat.com,
       linux-kernel@vger.kernel.org, agk@redhat.com
Subject: Re: [dm-devel] Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request()  to be called from interrupt context
Message-ID: <20061221184203.GD17199@kernel.dk>
References: <20061220134848.GF10535@kernel.dk> <20061220.125002.71083198.k-ueda@ct.jp.nec.com> <20061220184917.GJ10535@kernel.dk> <20061220.165549.39151582.k-ueda@ct.jp.nec.com> <20061221075305.GD17199@kernel.dk> <458ACB69.8000603@cs.wisc.edu> <458ACEB1.3030406@cs.wisc.edu> <20061221182432.GB17199@kernel.dk> <458AD2E0.40807@cs.wisc.edu> <458AD43C.3050603@cs.wisc.edu>
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On Thu, Dec 21 2006, Mike Christie wrote:
> Mike Christie wrote:
> > Jens Axboe wrote:
> >> On Thu, Dec 21 2006, Mike Christie wrote:
> >>> Or the block layer code could set up the clone too. elv_next_request
> >>> could prep a clone based on the orignal request for the driver then dm
> >>> would not have to worry about that part.
> >> It really can't, since it doesn't know how to allocate the clone
> >> request. I'd rather export this functionality as helpers.
> >>
> > 
> > What do you think about dm's plan to break up make_request into a
> > mapping function and in to the part the builds the bio into a request.
> > This would fit well with them being helpers and being able to allocate
> > the request from the correct context.
> > 
> > I see patches for that did not get posted, but I thought Joe and
> > Alasdair used to talk about that a lot and in the dm code I think there
> > is sill comments about doing it. Maybe the dm comments mentioned the
> > merge_fn, but I guess the merge_fn did not fit what they wanted to do or
> > something. I think Alasdair talked about this at one of his talks at OLS
> > or it was in a proposal for the kernel summit. I can dig up the mail if
> > you want.
> > 
> 
> Ignore that. The problem would be that we may not want to decide which
> path to use at map time.

Latter part, or both paragraphs? Dipping into ->make_request_fn() for
some parts do seem to make sense to me. It'll be cheaper than at
potential soft irq time (from elv_next_request()).

-- 
Jens Axboe

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