[dm-devel] Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

[Jens Axboe]

On Thu, Dec 21 2006, Mike Christie wrote:
> Jens Axboe wrote:
> > 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()).
> > 
> 
> I think we got crisscrossed.
> 
> The original idea but using your helper suggestion would have been this:
> 
> dm->make_request_fn(bio)
> {
> 	rq = __make_request(bio)
> 	if (this is a new request) {
> 		allocate clone from either a real device/path specific mempool() or a
> dm q mempool
> }
> 
> 
> dm->prep_fn(rq)
> {
> 	setup clone rq fields based on orig request fields.
> }
> 
> dm->request_fn(rq)
> {
> 	figure out which path to use;
> 	set rq->q;
> 	send cloned rq to real device;
> }

This'll work nicely, much better.

> The second idea based on Joe and Alasdair to break up make_request would
> just have been a more formal break up of the dm->make_request_fn above,
> because I thought your comment about not knowing how to allocate the
> clone request meant that we did not know which q's mempool to take the
> request from if we were going to take the cloned request from the real
> device/path's mempool. I guess this does not really matter since we can
> have just a dm q mempool of requests to use for cloned requests.

Either approach is fine with me. Note that you need to be careful with
foreign requests on a queue, see the elevator drain logic for barriers
and scheduler switching.

-- 
Jens Axboe