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. >> >> 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.
