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)
2)May be you have any other sound idea?
thank you.
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