[dm-devel] block: revert to using min_not_zero() when stacking chunk_sectors

[John Dorminy]

> If you're going to cherry pick a portion of a commit header please
> reference the commit id and use quotes or indentation to make it clear
> what is being referenced, etc.
Apologies.

> Quite the tangent just to setup an a toy example of say: thinp with 256K
> blocksize/chunk_sectors ontop of a RAID6 with a chunk_sectors of 128K
> and stripesize of 1280K.

I screwed up my math ... many apologies :/

Consider a thinp of chunk_sectors 512K atop a RAID6 with chunk_sectors 1280K.
(Previously, this RAID6 would be disallowed because chunk_sectors
could only be a power of 2, but 07d098e6bba removed this constraint.)
-With lcm_not_zero(), a full-device IO would be split into 2560K IOs,
which obviously spans both 512K and 1280K chunk boundaries.
-With min_not_zero(), a full-device IO would be split into 512K IOs,
some of which would span 1280k chunk boundaries. For instance, one IO
would span from offset 1024K to 1536K.
-With the hypothetical gcd_not_zero(), a full-device IO would be split
into 256K IOs, which span neither 512K nor 1280K chunk boundaries.

>
> To be clear, you are _not_ saying using lcm_not_zero() is correct.
> You're saying that simply reverting block core back to using
> min_not_zero() may not be as good as using gcd().

Assuming my understanding of chunk_sectors is correct -- which as per
blk-settings.c seems to be "a driver will not receive a bio that spans
a chunk_sector boundary, except in single-page cases" -- I believe
using lcm_not_zero() and min_not_zero() can both violate this
requirement. The current lcm_not_zero() is not correct, but also
reverting block core back to using min_not_zero() leaves edge cases as
above.

I believe gcd provides the requirement, but min_not_zero() +
disallowing non-power-of-2 chunk_sectors also provides the
requirement.

> > But it's possible I'm misunderstanding the purpose of chunk_sectors,
> > or there should be a check that the one of the two devices' chunk
> > sizes divides the other.
>
> Seriously not amused by your response, I now have to do damage control
> because you have a concern that you really weren't able to communicate
> very effectively.

Apologies.