Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks

[Linus Torvalds]

On Fri, Apr 15, 2022 at 3:58 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> BUG_ONs are harmful but problems that trigger them would be
> presummingly less penetrating to the user base; on the other hand,
> from my experience working with some testers (ordinary users), they
> ignore WARN_ON_ONCEs until the kernel crashes.

I don't understand your argument.

First you say that VM_BUG_ON() is only for VM developers.

Then you say "some testers (ordinary users) ignore WARN_ON_ONCEs until
the kernel crashes".

So which is it?

VM developers, or ordinary users?

Honestly, if a VM developer is ignoring a WARN_ON_ONCE() from the VM
subsystem, I don't even know what to say.

And for ordinary users, a WARN_ON_ONCE() is about a million times
better, becasue:

 - the machine will hopefully continue working, so they can report the warning

 - even when they don't notice them, distros tend to have automated
reporting infrastructure

That's why I absolutely *DETEST* those stupid BUG_ON() cases - they
will often kill the machine with nasty locks held, resulting in a
completely undebuggable thing that never gets reported.

Yes, you can be careful and only put BUG_ON() in places where recovery
is possible. But even then, they have no actual _advantages_ over just
a WARN_ON_ONCE.

                  Linus