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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