Re: [PATCH 01/12] generic/757: fix various bugs in this test

[Brian Foster <bfoster@redhat.com>]

On Fri, Nov 22, 2024 at 01:31:33PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 21, 2024 at 09:11:56AM -0500, Brian Foster wrote:
> > > I'm all for speeding up tests.  But relying on a unspecified side effect
> > > of an operation and then requiring a driver that implements that side
> > > effect without documenting that isn't really good practice.
> > > 
> > 
> > It's a hack to facilitate test coverage. It would obviously need to be
> > revisited if behavior changed sufficiently to break the test.
> > 
> > I'm not really sure what you're asking for wrt documentation. A quick
> > scan of the git history shows the first such commit is 65cc9a235919
> > ("generic/482: use thin volume as data device"), the commit log for
> > which seems to explain the reasoning.
> 
> A comment on _log_writes_init that it must only be used by dm-thin
> because it relies on the undocumented behavior that dm-trim zeroes
> all blocks discarded.
> 
> Or even better my moving the dm-think setup boilerplate into the log
> writes helpers, so that it gets done automatically.
> 

A related idea might be to incorporate your BLKZEROOUT fix so the core
tool is fundamentally correct, but then wrap the existing discard
behavior in a param or something that the dm-thin oriented tests can
pass to enable it as a fast zero hack/optimization.

But that all seems reasonable to me either way. I'm not sure that's
something I would have fully abstracted into the logwrites stuff
initially, but here we are ~5 years later and it seems pretty much every
additional logwrites test has wanted the same treatment. If whoever
wants to convert this newer test over wants to start by refactoring
things that way, that sounds like a welcome cleanup to me.

Brian