On 01/09/2023 18:18, Yang Shi wrote: > On Fri, Sep 1, 2023 at 9:13 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: >> >> On Thu, Aug 31, 2023 at 10:15:09AM -0700, Yang Shi wrote: >>> On Thu, Aug 31, 2023 at 12:57 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>> Let's talk about that in a bi-weekly MM session. (I proposed it as a >>>> topic for next week). >>>> >>>> As raised in another mail, we can then discuss >>>> * how we want to call this feature (transparent large pages? there is >>>> the concern that "THP" might confuse users. Maybe we can consider >>>> "large" the more generic version and "huge" only PMD-size, TBD) >>> >>> I tend to agree. "Huge" means PMD-mappable (transparent or HugeTLB), >>> "Large" means any order but less than PMD-mappable order, "Gigantic" >>> means PUD mappable. This should incur the least confusion IMHO. >> >> "Large" means any order > 0. The limitation to <= PMD_ORDER is simply >> because I don't want to go through the whole VM and fix all the places >> that assume that pmd_page() returns a head page. The benefit to doing so >> is quite small, and the work to achieve it is quite large. The amount of >> work needed should decrease over time as we convert more code to folios, >> so deferring it is the right decision today. > > Yeah, I agree. And we are on the same page. > >> >> But nobody should have the impression that large folios are smaller >> than PMD size, nor even less than or equal. Just like they shouldn't >> think that large folios depend on CONFIG_TRANSPARENT_HUGEPAGE. They do >> today, but that's purely an implementation detail that will be removed >> eventually. > > Yes, THP should be just a special case of large folio from page table > point of view (for example, PMD-mappable vs non-PMD-mappable). > >> >>>> I think there *really* has to be a way to disable it for a running >>>> system, otherwise no distro will dare pulling it in, even after we >>>> figured out the other stuff. >>> >>> TBH I really don't like to tie large folio to THP toggles. THP >>> (PMD-mappable) is just a special case of LAF. The large folio should >>> be tried whenever it is possible ideally. But I do agree we may not be >>> able to achieve the ideal case at the time being, and also understand >>> the concern about regression in early adoption, so a knob that can >>> disable large folio may be needed for now. But it should be just a >>> simple binary knob (on/off), and should not be a part of kernel ABI >>> (temporary and debugging only) IMHO. >> >> Best of luck trying to remove it after you've shipped it ... we've >> never been able to remove any of the THP toggles, only make them more >> complicated. > > Fingers crossed... and my point is we should try to avoid making > things more complicated. It may be hard... > >> >>> One more thing we may discuss is whether huge page madvise APIs should >>> take effect for large folio or not. >> >> They already do for file large folios; we listen to MADV_HUGEPAGE and >> attempt to allocate PMD_ORDER folios for faults. > > OK, file folio may be simpler than anonymous. For anonymous folio, > there may be two potential cases depending on our choice: > > Tie large folio to THP knobs: > MADV_HUGEPAGE - large folio if THP is on/no large folio if THP is off > MADV_NOHUGEPAGE - no large folio > > Not tie large folio to THP knob: > MADV_HUGEPAGE - always large folio > MADV_NOHUGEPAGE - shall create large folio? > In my mind, the debate on how LAF and MADV_NOHUGEPAGE should interact is concluded; David has explained a QEMU live migration use case, which would break if a LAF was allocated for a VMA with MADV_NOHUGEPAGE (see [1]). Given LAF and THP controls must be tied together at MADV_NOHUGEPAGE as a minimum, then for me it makes most sense to expose LAF to user space as a generalization of THP rather than a separate, independent feature. And if taking such a route, Huang Ying's suggestion at [2] sounds like a good starting point. Anyway, let's discuss in the mm meeting as David requested. [1] https://lore.kernel.org/linux-mm/b936041c-08a7-e844-19e7-eafc4ddf63b9@xxxxxxxxxx/ [2] https://lore.kernel.org/linux-mm/87v8dg6lfu.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
