linux-kernel - Re: bi_end

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Date:	Tue, 31 Jul 2007 09:29:54 +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:
> 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.

-- 
Jens Axboe

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