On Fri, May 30, 2025 at 10:07:11AM +0100, Ryan Roberts wrote: > On 30/05/2025 09:52, David Hildenbrand wrote: > > 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 think it's a bit reductive to suggest that enabled=madvise means all madvise > calls though. I don't think anyone would suggest MADV_DONTNEED should be ignored > if enabled=never. MADV_COLLAPSE just happens to be implemented on top of the THP > logic. But it's a different feature in my view. No I absolutely take your point, and indeed this is very reductive, but I think that's a product of this interface being... sub-optimal. if you dig into the docs for instance it's explicit about that referring to MADV_[NO]HUGEPAGE. But, as a user/sys-admin, I'd definitely find that surprising. I think the intent of 'never' people is 'THP bad I don't want it' for whatever reason that might be the case. > > >> > >> 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, > > Yes agreed there! > > >> 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. > > > > OK fair enough. Just giving my 2 cents. > > Your input is very welcome! We have made a mess here so it's good to talk it through.
