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

[John Hubbard <jhubbard@nvidia.com>]

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:

* The only time you'll be flooded with WARN_ON() messages is when *two*
things happen at once:

    a) Something that used to completely crash the machine (a VM_BUG_ON
       condition) happens, and

    b) You're in a loop and it keeps on happening. Yes, in -mm, that does
       happen a lot (per-page loops, for example), but still.

* It's *so* easy to miss a WARN_ON_ONCE(). We don't want that, not for a
critical failure case that used to be a VM_BUG_ON().


thanks,
-- 
John Hubbard