Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[David Hildenbrand <david@redhat.com>]

On 25.06.25 09:42, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 09:36:53AM +0200, David Hildenbrand wrote:
>> On 25.06.25 09:30, Lorenzo Stoakes wrote:
>>> I _guarantee_ you that's what nearly everybody except a handful of people will
>>> expect.
>>
>> I know, See my other mail, the problem is rather if there is no somebody
>> relying on never+MADV_COLLAPSE from doing the MADV_COLLAPSE-documented
>> thing.
>>
>> It's a mess.
> 
> Well now we have an almost philosophical debate - we have different sets of
> users, 99% of whom believe the uAPI is X, and 1% of whom believe it is Y.
> 
> Now what is the uAPI? What is 'breaking userspace'? :)

Yeah, that's why I mentioned that I think we broke "something" when we 
changed the semantics. But that breakage probably only affects real 
corner cases (debugging, customer workarounds).

I think the whole use case of using MADV_COLLAPSE to completely control 
THP allocation in a system is otherwise pretty hard to achieve, if there 
is no other way to tame THP allocation through page faults+khugepaged.

Mess.

-- 
Cheers,

David / dhildenb