[dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking

[Ming Lei]

On Thu, Dec 03, 2020 at 08:27:38AM -0800, Keith Busch wrote:
> On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote:
> > On Wed, Dec 02 2020 at 10:26pm -0500,
> > Ming Lei <ming.lei at redhat.com> wrote:
> > 
> > > I understand it isn't related with correctness, because the underlying
> > > queue can split by its own chunk_sectors limit further. So is the issue
> > > too many further-splitting on queue with chunk_sectors 8? then CPU
> > > utilization is increased? Or other issue?
> > 
> > No, this is all about correctness.
> > 
> > Seems you're confining the definition of the possible stacking so that
> > the top-level device isn't allowed to have its own hard requirements on
> > IO sizes it sends to its internal implementation.  Just because the
> > underlying device can split further doesn't mean that the top-level
> > virtual driver can service larger IO sizes (not if the chunk_sectors
> > stacking throws away the hint the virtual driver provided because it
> > used lcm_not_zero).
> 
> I may be missing something obvious here, but if the lower layers split
> to their desired boundary already, why does this limit need to stack?
> Won't it also work if each layer sets their desired chunk_sectors
> without considering their lower layers? The commit that initially
> stacked chunk_sectors doesn't provide any explanation.

There could be several reasons:

1) some limits have to be stacking, such as logical block size, because
lower layering may not handle un-aligned IO

2) performance reason, if every limits are stacked on topmost layer, in
theory IO just needs to be splitted in top layer, and not need to be
splitted further from all lower layer at all. But there should be exceptions
in unusual case, such as, lowering queue's limit changed after the stacking
limits are setup.

3) history reason, bio splitting is much younger than stacking queue
limits.

Maybe others?


Thanks,
Ming