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
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