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

[Yu Zhao]

On Fri, Apr 15, 2022 at 3:43 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 15, 2022 at 2:32 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > We could create a new MM-developer-only assertion.  Might even call it
> > MM_BUG_ON().  With compile-time enablement but perhaps not a runtime
> > switch.
>
> .. or VM_BUG_ON() could just become a WARN_ON_ONCE().
>
> Which it should be anyway - since the code has to be written to
> continue after that BUG_ON() anyway.
>
> There is absolutely _zero_ advantage to killing the machine. If you
> want to be notified about "this must not happen", then WARN_ON_ONCE()
> is the right thing to use.
>
> BUG_ON() is basically always the wrong thing to do.

Not trying to start a meta discussion, just my two cents:

This is a typical trolley problem: for the greater good, do we want to
inflict more pain on a small group of users running experimental
kernels so that they'd come back and yell at us quicker and louder?
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'll let Justin chime in on Fedora's take on CONFIG_DEBUG_VM. I bet
it's intended to crash the kernel.