2011/10/9 Mike Snitzer <snitzer@xxxxxxxxxx>: > On Sat, Oct 08 2011 at 7:02am -0400, > Shaohua Li <shli@xxxxxxxxxx> wrote: > >> Looks the dm request based flush logic is broken. >> >> saved_make_request_fn >> __make_request >> blk_insert_flush >> but blk_insert_flush doesn't put the original request to list, instead, the >> q->flush_rq is in list. >> then >> dm_request_fn >> blk_peek_request >> dm_prep_fn >> clone_rq >> map_request >> blk_insert_cloned_request >> so q->flush_rq is cloned, and get dispatched. but we can't clone q->flush_rq >> and use it to do flush. map_request even could assign a different blockdev to >> the cloned request. > > You haven't explained why cloning q->flush_rq is broken. What is the > problem with map_request changing the blockdev? For the purposes of > request-based DM the flush machinery has already managed the processing > of the flush at the higher level request_queue. hmm, looks I overlook the issue. cloned flush_rq has some problems but can be fixed. 1. it doesn't set requet->bio, request->bio_tail 2. REQ_CLONE_MASK should set REQ_FLUSH_SEQ > By the time request-based DM is cloning a flush request it really has no > need to reenter the flush machinery (even though Tejun wants it to -- > but in practice it doesn't buy us anything because we never stack > request-based DM at the moment. Instead it showcases how brittle this > path is). if there is no benefit, we'd better not clone a flush request. Clearing flush bit and set it to cloned request is more clean and avoid unnecessary overhead/complexity. Thanks, Shaohua -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
