The kernel-development process routinely absorbs large changes to
fundamental subsystems and still produces stable releases every nine or ten
weeks. On occasion, though, the development community’s luck runs out.
The per-VMA locking work that went into the
6.4 release is a case in point; it looked like a well-tested change that
improved page-fault scalability. There turned out to be a few demons
hiding in that code, though, that made life difficult for early adopters of
the 6.4 kernel.

Source: LWN.net – [$] Stabilizing per-VMA locking