On 09:29 Втр 31 Июл , 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" ?
>
> --
> Jens Axboe
>
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