Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking

[Ming Lei <ming.lei@redhat.com>]

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@redhat.com> wrote:
> 
> > On Tue, Dec 01, 2020 at 11:07:09AM -0500, Mike Snitzer wrote:
> > > commit 22ada802ede8 ("block: use lcm_not_zero() when stacking
> > > chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must
> > > reflect the most limited of all devices in the IO stack.
> > > 
> > > Otherwise malformed IO may result. E.g.: prior to this fix,
> > > ->chunk_sectors = lcm_not_zero(8, 128) would result in
> > > blk_max_size_offset() splitting IO at 128 sectors rather than the
> > > required more restrictive 8 sectors.
> > 
> > What is the user-visible result of splitting IO at 128 sectors?
> 
> The VDO dm target fails because it requires IO it receives to be split
> as it advertised (8 sectors).

OK, looks VDO's chunk_sector limit is one hard constraint, even though it
is one DM device, so I guess you are talking about DM over VDO?

Another reason should be that VDO doesn't use blk_queue_split(), otherwise it
won't be a trouble, right?

Frankly speaking, if the stacking driver/device has its own hard queue limit
like normal hardware drive, the driver should be responsible for the splitting.

> 
> > 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

I just don't know this story, thanks for your clarification.

As I mentioned above, if the stacking driver has its own hard queue
limit, it should be the driver's responsibility to respect it via
blk_queue_split() or whatever.


Thanks,
Ming