Re: [PATCH v1] mm/gup: remove (VM_)BUG_ONs

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Jun 06, 2025 at 12:13:27PM +0200, Michal Hocko wrote:
> On Fri 06-06-25 11:01:18, David Hildenbrand wrote:
> > On 06.06.25 10:31, Michal Hocko wrote:
> [...]
> > > Turning them into VM_WARN_ON
> > > should be reasonably safe as they are not enabled in production
> > > environment anyway so we cannot really rely on those. Having them in
> > > WARN form would be still useful for debugging and those that really need
> > > a crash dump while debugging can achieve the same result.
> >
> > One question is if we should be VM_WARN_ON vs. VM_WARN_ON_ONCE ...
>
> *WARN_ONCE ha a very limited use from code paths which are generally
> shared by many callers. You just see one and then nothing. Some time ago
> we have discussed an option to have _ONCE per call trace but I haven't
> see any follow up.
>
> Anyway starting without _ONCE seems like safer option because we are not
> losing potentially useful debugging information. Afterall this is
> debugging only thing. But no strong position on my side.
>
> > VM_BUG_ON is essentially a "once" thing so far, but then, we don't continue
> > ... so probably most should be _ONCE.
> >
> > >
> > > So while I agree that many of them could be dropped or made more clear
> > > those could be dealt with after a mass move. An advantage of this would
> > > be that we can drop VM_BUG_ON* and stop new instances from being added.
> >
> > As a first step we could probably just #define them to go to the
> > VM_WARN_ON_* variants and see what happens.
>
> You meand VM_BUG_ON expand to VM_WARN_ON by default?

Sorry to interject in the conversation, but to boldly throw my two English pence
into the mix:

As the "king of churn" (TM) you'll not be surprised to hear that I'm in favour
of us just doing a big patch and convert all VM_BUG_ON() -> VM_WARN_ON_ONCE()
and remove VM_BUG_ON*().

Pull the band-aid off... I think better than a #define if this indeed what you
meant David.

But of course, you'd expect me to have this opinion ;)

>
> --
> Michal Hocko
> SUSE Labs

Cheers, Lorenzo