Re: fuzzing bcachefs with dm-flakey

[Kent Overstreet <kent.overstreet@linux.dev>]

On Tue, May 30, 2023 at 05:00:39PM -0400, Mikulas Patocka wrote:
> I'd like to know how do you want to do coverage analysis? By instrumenting 
> each branch and creating a test case that tests that the branch goes both 
> ways?

Documentation/dev-tools/gcov.rst. The compiler instruments each branch
and then the results are available in debugfs, then the lcov tool
produces annotated source code as html output.

> I know that people who write spacecraft-grade software do such tests, but 
> I can't quite imagine how would that work in a filesystem.
> 
> "grep -w if fs/bcachefs/*.[ch] | wc -l" shows that there are 5828 
> conditions. That's one condition for every 15.5 lines.

Most of which are covered by existing tests - but by running the
existing tests with code coverage analylis we can see which branches the
tests aren't hitting, and then we add fault injection points for those.

With fault injection we can improve test coverage a lot without needing
to write any new tests (or simple ones, for e.g. init/mount errors)