[rwmj]

It's a great feature, but it had a few unintended side-effects:

https://lwn.net/Articles/819834/ "Blocking userfaultfd() kernel-fault handling" (patches to fix a security issue, more detail in https://duasynt.com/blog/linux-kernel-heap-spray)

https://lwn.net/Articles/849876/ "Patching until the COWs come home" (complicated interaction with copy-on-write)