bcachefs docs: SubmittingPatches.rst

Submitting patches to bcachefs:

===============================

Patches must be tested before being submitted, either with the xfstests suite

[0], or the full bcachefs test suite in ktest [1], depending on what's being

touched. Note that ktest wraps xfstests and will be an easier method to running

it for most users; it includes single-command wrappers for all the mainstream

in-kernel local filesystems.

Patches will undergo more testing after being merged (including

lockdep/kasan/preempt/etc. variants), these are not generally required to be

run by the submitter - but do put some thought into what you're changing and

which tests might be relevant, e.g. are you dealing with tricky memory layout

work? kasan, are you doing locking work? then lockdep; and ktest includes

single-command variants for the debug build types you'll most likely need.

The exception to this rule is incomplete WIP/RFC patches: if you're working on

something nontrivial, it's encouraged to send out a WIP patch to let people

know what you're doing and make sure you're on the right track. Just make sure

it includes a brief note as to what's done and what's incomplete, to avoid

confusion.

Rigorous checkpatch.pl adherence is not required (many of its warnings are

considered out of date), but try not to deviate too much without reason.

Focus on writing code that reads well and is organized well; code should be

aesthetically pleasing.

CI:

===

Instead of running your tests locally, when running the full test suite it's

prefereable to let a server farm do it in parallel, and then have the results

in a nice test dashboard (which can tell you which failures are new, and

presents results in a git log view, avoiding the need for most bisecting).

That exists [2], and community members may request an account. If you work for

a big tech company, you'll need to help out with server costs to get access -

but the CI is not restricted to running bcachefs tests: it runs any ktest test

(which generally makes it easy to wrap other tests that can run in qemu).

Other things to think about:

============================

- How will we debug this code? Is there sufficient introspection to diagnose

  when something starts acting wonky on a user machine?

  We don't necessarily need every single field of every data structure visible

  with introspection, but having the important fields of all the core data

  types wired up makes debugging drastically easier - a bit of thoughtful

  foresight greatly reduces the need to have people build custom kernels with

  debug patches.

  More broadly, think about all the debug tooling that might be needed.

- Does it make the codebase more or less of a mess? Can we also try to do some

  organizing, too?

- Do new tests need to be written? New assertions? How do we know and verify

  that the code is correct, and what happens if something goes wrong?

  We don't yet have automated code coverage analysis or easy fault injection -

  but for now, pretend we did and ask what they might tell us.

  Assertions are hugely important, given that we don't yet have a systems

  language that can do ergonomic embedded correctness proofs. Hitting an assert

  in testing is much better than wandering off into undefined behaviour la-la

  land - use them. Use them judiciously, and not as a replacement for proper

  error handling, but use them.

- Does it need to be performance tested? Should we add new peformance counters?

  bcachefs has a set of persistent runtime counters which can be viewed with

  the 'bcachefs fs top' command; this should give users a basic idea of what

  their filesystem is currently doing. If you're doing a new feature or looking

  at old code, think if anything should be added.

- If it's a new on disk format feature - have upgrades and downgrades been

  tested? (Automated tests exists but aren't in the CI, due to the hassle of

  disk image management; coordinate to have them run.)

Mailing list, IRC:

==================

Patches should hit the list [3], but much discussion and code review happens on

IRC as well [4]; many people appreciate the more conversational approach and

quicker feedback.

Additionally, we have a lively user community doing excellent QA work, which

exists primarily on IRC. Please make use of that resource; user feedback is

important for any nontrivial feature, and documenting it in commit messages

would be a good idea.

[0]: git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git

[1]: https://evilpiepirate.org/git/ktest.git/

[2]: https://evilpiepirate.org/~testdashboard/ci/

[3]: linux-bcachefs@vger.kernel.org

[4]: irc.oftc.net#bcache, #bcachefs-dev

   :numbered:

   CodingStyle

   SubmittingPatches

   errorcodes

L:	linux-bcachefs@vger.kernel.org

S:	Supported

C:	irc://irc.oftc.net/bcache

P:      Documentation/filesystems/bcachefs/SubmittingPatches.rst

T:	git https://evilpiepirate.org/git/bcachefs.git

F:	fs/bcachefs/

F:	Documentation/filesystems/bcachefs/