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

[David Hildenbrand <david@redhat.com>]

On 06.06.25 20:06, Lorenzo Stoakes wrote:
> On Fri, Jun 06, 2025 at 10:57:44AM -0700, John Hubbard wrote:
>> On 6/6/25 4:04 AM, Lorenzo Stoakes wrote:
>>> On Fri, Jun 06, 2025 at 12:28:28PM +0200, David Hildenbrand wrote:
>>>> On 06.06.25 12:19, Lorenzo Stoakes wrote:
>>>>> 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:
>>>>>> [...]
>>> So to me the only assessment needed is 'do we want to warn on this or not?'.
>>>
>>> And as you say, really WARN_ON_ONCE() seems appropriate, because nearly always
>>> we will get flooded with useless information.
>>>
>>
>> As yet another victim of such WARN_ON() floods at times, I've followed
>> this thread with great interest. And after reflecting on it a bit, I believe
>> that, surprisingly enough, WARN_ON() is a better replacement for VM_BUG_ON()
>> than WARN_ON_ONCE(), because:
> 
> Right, these shouldn't be happening _at all_.
 > > I'm easy on this point, I'd say in that case VM_WARN_ON() is the most
> _conservative_ approach, since these are things that must not happen, and
> so it's not unreasonable to fail to repress repetitions of the 'impossible'
> :)
> 
> But I get the general point about ...WARN_ON_ONCE() avoiding floods.
> 
> David, what do you think?

Well, in this patch here I deliberately want _ONCE for the unpin sanity 
checks. Because if they start happening (IOW, now after 5 years observed 
for the first time?) I *absolutely don't* want to get flooded and 
*really* figure out what is going on by seeing what else failed.

And crashing on VM_BUG_ON() and not observing anything else was also not 
particularly helpful :)

Because ... they shouldn't be happening ...

(well, it goes back to my initial point about requiring individual 
decisions etc ...)

Not sure what's best now in the general case, in the end I don't care 
that much.

Roll a dice? ;)

-- 
Cheers,

David / dhildenb