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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