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. > -- > Cheers, > > David / dhildenb >
