linux-kernel - Re: bi_end

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Date:	Tue, 31 Jul 2007 09:44:41 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: bi_end_io question

On Tue, Jul 31 2007, Dmitry Monakhov wrote:
> On 09:29 ?????? 31 ??????     , Jens Axboe wrote:
> > On Tue, Jul 31 2007, Dmitry Monakhov wrote:
> > > I want implement some sort of snapshot dev.
> > > In order to do this i've replaced original q->make_request_fn
> > > with specific one which cowed all write requests.
> > > My first attempt looks like this:
> > > static int mysnap_make_request(request_queue_t *q, struct bio *bio)
> > > {
> > > 	struct bio cl_bio;
> > > 	make_request_fn *fn = get_original_make_fn(q);
> > > 	if(bio->bi_rw & (1 << BIO_RW){
> > > 	/* This is write request, so we have to cow it first*/
> > > 		cl_bio = clone_bio(bio);
> > > 		submit_bio(READ, &cl_bio);
> > > 		wait_for_completion(&complete); 
> > > 		save_cowed_bio(&cl_bio);
> > > 		/* At this time old data has successfully saved in cow
> > > 		 * area so we may safely perform original request */
> > > 	}
> > > 	/* call original make_request_fn function for bio*/
> > > 	return fn(q, bio);
> > > }
> > > But this implementation has significance performance drawback because
> > > of waiting for read request completion.
> > > 
> > > So i want use a-sync implementation:
> > > static int my_make_original_request(struct bio *bio, unsigned int bytes_done,
> > >                                int err)
> > > {
> > > 	if(bio->bi_size == 0) {
> > > 		/* restore original request info */
> > > 		struct pending_request*  pr = bio->bi_private;
> > > 		/* make original write request */
> > > 		ret = pr->fn(pr->q, pr->bio);
> > > 		/* error handling logic here*/
> > > 	}
> > > }
> > > static int mysnap_make_request_v2(request_queue_t *q, struct bio *bio)
> > > {
> > > 	struct bio cl_bio;
> > > 	make_request_fn *fn = get_original_make_fn(q);
> > > 	if(bio->bi_rw & (1 << BIO_RW){
> > > 	/* This is write request, so we have to cow it first*/
> > > 		cl_bio = clone_bio(bio);
> > > 		cl_bio->bi_private = store_request(fn, q, bio)
> > > 		cl_bio->bi_end_io = my_make_original_request;
> > > 		/* after read request ended original write request 
> > > 		 * will be queued */
> > > 		submit_bio(READ, &cl_bio, q);
> > > 		/* We dont have to wait submitted bio here any more,
> > > 		 * because write request will be automaticaly queued
> > > 		 * by cl_bio->bi_end_io callcack.
> > > 		 * All job has been done at this moment.
> > > 		 */
> > > 		return 0;
> > > 	}
> > > 	/* call original make_request_fn function for bio*/
> > > 	return fn(q, bio);
> > > }
> > > My question is following:
> > > 1)Can i safely call make_request_fn from ->bi_end_io callback 
> > >   (as it done in my_make_original_request function)
> > 
> > No, make_request_fn must be called from process context.
> > 
> > > 2)May be you have any other sound idea?
> > 
> > Allocate a real clone, if you want to do handle it async.
> what do you mean by "real clone" ?

One that isn't allocated on the stack.

-- 
Jens Axboe

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