On 09.08.22 20:27, Linus Torvalds wrote: > I'm still reading through this, but > > STOP DOING THIS > > On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> + VM_BUG_ON(!is_cow_mapping(vma->vm_flags)); > > Using BUG_ON() for debugging is simply not ok. > > And saying "but it's just a VM_BUG_ON()" does not change *anything*. > At least Fedora enables that unconditionally for normal people, it is > not some kind of "only VM people do this". > > Really. BUG_ON() IS NOT FOR DEBUGGING. I totally agree with BUG_ON ... but if I get talked to in all-caps on a Thursday evening and feel like I just touched the forbidden fruit, I have to ask for details about VM_BUG_ON nowadays. VM_BUG_ON is only active with CONFIG_DEBUG_VM. ... which indicated some kind of debugging at least to me. I *know* that Fedora enables it and I *know* that this will make Fedora crash. I know why Fedora enables this debug option, but it somewhat destorys the whole purpose of VM_BUG_ON kind of nowadays? For this case, this condition will never trigger and I consider it much more a hint to the reader that we can rest assured that this condition holds. And on production systems, it will get optimized out. Should we forbid any new usage of VM_BUG_ON just like we mostly do with BUG_ON? > > Stop it. Now. > > If you have a condition that must not happen, you either write that > condition into the code, or - if you are convinced it cannot happen - > you make it a WARN_ON_ONCE() so that people can report it to you. I can just turn that into a WARN_ON_ONCE() or even a VM_WARN_ON_ONCE(). -- Thanks, David / dhildenb
