Add all read-only compatible zpool features to grub2 compatibility

[usaleem-ix]

Motivation and Context

GRUB opens the boot-pool in read-only mode, but not all read-only compatible zpool
features are listed in grub2 compatibility file.

Description

GRUB opens the boot pool in read-only mode. All read-only compatible features for zpool
can be enabled and added to grub2 compatibility, as GRUB does not open the boot-pool
for write.

How Has This Been Tested?

Tested by creating a boot-pool with all read-only compatible zpool features enabled and
booting from the boot-pool.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[rincebrain]

#15261 notes that any feature that enables extensible_dataset appears to break at least some versions of grub if there's literally any snapshot ever on the root dataset, and it does not unbreak if you destroy the snapshot.

So this should probably be reverted.

(I don't, personally, boot ZFS with GRUB, so this is just my summary from trying to digest those bug reports, but it seems like we should probably do something about breaking things like this...)

[usaleem-ix]

@rincebrain Since this PR was opened after #15261 was reported, it seems less likely this would be the root cause. I quickly checked and this commit is not present in 2.1.x and 2.2.0, which the issue is describing. It's present from 2.2.1 onwards.

extensible_dataset feature was added in grub2 compatibility list when it was first introduced 3 years ago. We may proceed to remove extensible_dataset and all features that depend on extensible_dataset, but reverting this one would not cover all features that can enable extensible_dataset.

[rincebrain]

Sure, sorry, I wasn't trying to say just reverting this would be sufficient, just that it was a concrete example of the concerns I voiced earlier in the PR about grub breaking sometimes on some versions. Wondering if we might want a versioned grub compat in addition to the "lowest denominator" functionality, given that things like this are probably going to keep happening over time, and people are going to want both "as many features as possible" and "this always works with grub" as options...

…

On Mon, Feb 12, 2024, 12:12 PM Umer Saleem ***@***.***> wrote: @rincebrain <https://github.com/rincebrain> Since this PR was opened after #15261 <#15261> was reported, it seems less likely this would be the root cause. I quickly checked and this commit is not present in 2.1.x and 2.2.0, which the issue is describing. It's present from 2.2.1 onwards. extensible_dataset feature was added in grub2 compatibility list when it was first introduced 3 years ago. We may proceed to remove extensible_dataset and all features that depend on extensible_dataset, but reverting this one would not cover all features that can enable extensible_dataset. — Reply to this email directly, view it on GitHub <#15459 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABUI7MVWISHRY4QLJI2X4DYTJEO7AVCNFSM6AAAAAA6SLZTC6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSMZZGE3TGOJSGM> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[usaleem-ix]

Adding a second compatibility file with minimum set of features required, seems more reasonable to me.

[behlendorf]

Then perhaps a new cmd/zpool/compatibility.d/grub2-minimal compatibility file with just the bare essentials.

[rincebrain]

I almost feel like the reverse might be better, where "grub2" means "every version of grub2 works with this" and "grub2-features" means "this is the most features we know of any grub2 version supporting", since people have, historically, used grub2 to try to mean "as many features as we can reliably enable without breaking boot, and no more", so moving that concept to a new name would require changing lots of expectations, whereas a new featureful one would be adding a new option while avoiding changing older expectations for this.

[usaleem-ix]

#15909