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

[David Hildenbrand <david@redhat.com>]

On 30.05.25 10:47, Lorenzo Stoakes wrote:
> On Fri, May 30, 2025 at 10:44:36AM +0200, David Hildenbrand wrote:
>> On 30.05.25 10:04, Ryan Roberts wrote:
>>> On 29/05/2025 09:23, Baolin Wang wrote:
>>>> As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore
>>>> the system-wide anon/shmem THP sysfs settings, which means that even though
>>>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still
>>>> attempt to collapse into a anon/shmem THP. This violates the rule we have
>>>> agreed upon: never means never. This patch set will address this issue.
>>>
>>> This is a drive-by comment from me without having the previous context, but...
>>>
>>> Surely MADV_COLLAPSE *should* ignore the THP sysfs settings? It's a deliberate
>>> user-initiated, synchonous request to use huge pages for a range of memory.
>>> There is nothing *transparent* about it, it just happens to be implemented using
>>> the same logic that THP uses.
>>>
>>> I always thought this was a deliberate design decision.
>>
>> If the admin said "never", then why should a user be able to overwrite that?
>>
>> The design decision I recall is that if VM_NOHUGEPAGE is set, we'll ignore
>> that. Because that was set by the app itself (MADV_NOHUEPAGE).
>>
> 
> I'm with David on this one.
> 
> I think it's principal of least surprise - to me 'never' is pretty
> emphatic, and keep in mind the other choices are 'always' and...  'madvise'
> :) which explicitly is 'hey only do this if madvise tells you to'.
> 
> I'd be rather surprised if I hadn't set madvise and a user uses madvise to
> in some fashion override the never.
> 
> I mean I think we all agree this interface is to use a technical term -
> crap - and we need something a lot more fine-grained and smart, but I think
> given the situation we're in we should make it at least as least surprising
> as possible.

Yes. If you configure "never" you are supposed to suffer, consistently.

-- 
Cheers,

David / dhildenb