Please revert LTS kernel change that will break ZFS for Buster point releases

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Please revert LTS kernel change that will break ZFS for Buster point releases

Mo Zhou
control: severity -1 grave

Dear kernel maintainers,

Buster will be released with 4.19.37 kernel. That's fine
and it doesn't break ZFS. However, the changes introduced
in 4.19.38 and linux 5.0 break ZFS. That means the current
0.7.12-2 will fail to build everywhere after the first
Buster point release (with kernel version bump).
A foreseeable stable RC is grave enough.

Upstream has made a compromise to disable SIMD for kernels
that received the breaking change:
 
https://github.com/zfsonlinux/zfs/commit/e22bfd814960295029ca41c8e116e8d516d3e730

Based on these, we Debian ZoL maintainers have several
possible choices:

1. unblock 0.7.13-1 (see rmadison -S zfs-linux)
   (it contains the above patch)
2. revert debdiff(0.7.12-2,0.7.12-5), apply the upstream
   patch and upload 0.7.12-6 through t-p-u.
3. ask kernel maintainers to revert problematic commits

Freeze policy makes it difficult for the release team to
accept solution 1. Solution 2 is able to eliminate bugs
but I doubt how useful a SIMD-less ZFS is. Compared to
the others, solution 3 is the best solution because there
won't be any future breakage or significant performance loss.

My position is solution 3, as it's the ***LTS KERNEL UPDATE***
that introduced breaking change breaks ZFS 0.7.12-2.
It's not a bug of ZFS 0.7.12-2 at all.

Debian ZFS users are sensitive because I and Aron often
receive private user reports and prodding mails. That means
reverting the problematic kernel commit is beneficial to
our users. Our priorities are our users and free software,
NOT to stick to the problematic breaking LTS update.

I believe this is a kernel bug. Instead of submitting
a grave RC for the 10.1 release, we'd better sort it out
right now before the Buster release.

Thanks,
Mo