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

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

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?

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?

> 
> And since commit 07d098e6bbad ("block: allow 'chunk_sectors' to be
> non-power-of-2") care must be taken to properly stack chunk_sectors to
> be compatible with the possibility that a non-power-of-2 chunk_sectors
> may be stacked. This is why gcd() is used instead of reverting back
> to using min_not_zero().

I guess gcd() won't be better because gcd(a,b) is <= max(a, b), so bio
size is decreased much with gcd(a, b), and IO performance should be affected.
Maybe worse than min_not_zero(a, b) which is often > gcd(a, b).


Thanks,
Ming

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