On Mon, Nov 13, 2017 at 07:57:45AM -0800, Tejun Heo wrote: > Hello, > > On Mon, Nov 13, 2017 at 03:13:48AM -0800, Tejun Heo wrote: > > You're right. If we wanna take this approach, we need to keep the > > throttled flag while cloning. The clearing part is still correct tho. > > Without that, I get 1/4 bw limit enforced. Hmm... I'm not quite sure > > where that 1/4 is coming from tho. Will investigate more. > > Okay, this is because when we spiit, the split bio is the first part > which gets issued and then the orignal bio is wound forward and > requeued. So, for the splits, the original bio is the one which gets > trimmed in the front and requeued, so not clearing BIO_THROTTLED is > enough. I think we should still copy BIO_THROTTLED on clones so that > we don't get suprises w/ other bio drivers.
I'm not sure how you are going to make this correct. The mechanism is very fragile. So for example, 'q->make_request_fn(q, bio)' could just queue the bio somewhere and handle in other context (both dm and md do this). The bio will be called again with generic_make_request. In this case, the second time shouldn't throttle the bio. The bio could be called again with generic_make_request but with bdev changed. In this case, the second time should throttle the bio (against the new bdev). There are a lot of different usages of bio. I'd rather not depend on generic_make_request dispatches new bio immediately. That's why I add a bdev in my patch. Thanks, Shaohua
