On Wed, Apr 10, 2024 at 04:30:41PM +0000, Christophe Leroy wrote: > > > Le 10/04/2024 à 17:28, Peter Xu a écrit : > > On Tue, Apr 09, 2024 at 08:43:55PM -0300, Jason Gunthorpe wrote: > >> On Fri, Apr 05, 2024 at 05:42:44PM -0400, Peter Xu wrote: > >>> In short, hugetlb mappings shouldn't be special comparing to other huge pXd > >>> and large folio (cont-pXd) mappings for most of the walkers in my mind, if > >>> not all. I need to look at all the walkers and there can be some tricky > >>> ones, but I believe that applies in general. It's actually similar to what > >>> I did with slow gup here. > >> > >> I think that is the big question, I also haven't done the research to > >> know the answer. > >> > >> At this point focusing on moving what is reasonable to the pXX_* API > >> makes sense to me. Then reviewing what remains and making some > >> decision. > >> > >>> Like this series, for cont-pXd we'll need multiple walks comparing to > >>> before (when with hugetlb_entry()), but for that part I'll provide some > >>> performance tests too, and we also have a fallback plan, which is to detect > >>> cont-pXd existance, which will also work for large folios. > >> > >> I think we can optimize this pretty easy. > >> > >>>> I think if you do the easy places for pXX conversion you will have a > >>>> good idea about what is needed for the hard places. > >>> > >>> Here IMHO we don't need to understand "what is the size of this hugetlb > >>> vma" > >> > >> Yeh, I never really understood why hugetlb was linked to the VMA.. The > >> page table is self describing, obviously. > > > > Attaching to vma still makes sense to me, where we should definitely avoid > > a mixture of hugetlb and !hugetlb pages in a single vma - hugetlb pages are > > allocated, managed, ... totally differently. > > > > And since hugetlb is designed as file-based (which also makes sense to me, > > at least for now), it's also natural that it's vma-attached. > > > >> > >>> or "which level of pgtable does this hugetlb vma pages locate", > >> > >> Ditto > >> > >>> because we may not need that, e.g., when we only want to collect some smaps > >>> statistics. "whether it's hugetlb" may matter, though. E.g. in the mm > >>> walker we see a huge pmd, it can be a thp, it can be a hugetlb (when > >>> hugetlb_entry removed), we may need extra check later to put things into > >>> the right bucket, but for the walker itself it doesn't necessarily need > >>> hugetlb_entry(). > >> > >> Right, places may still need to know it is part of a huge VMA because we > >> have special stuff linked to that. > >> > >>>> But then again we come back to power and its big list of page sizes > >>>> and variety :( Looks like some there have huge sizes at the pgd level > >>>> at least. > >>> > >>> Yeah this is something I want to be super clear, because I may miss > >>> something: we don't have real pgd pages, right? Powerpc doesn't even > >>> define p4d_leaf(), AFAICT. > >> > >> AFAICT it is because it hides it all in hugepd. > > > > IMHO one thing we can benefit from such hugepd rework is, if we can squash > > all the hugepds like what Christophe does, then we push it one more layer > > down, and we have a good chance all things should just work. > > > > So again my Power brain is close to zero, but now I'm referring to what > > Christophe shared in the other thread: > > > > https://github.com/linuxppc/wiki/wiki/Huge-pages > > > > Together with: > > > > https://lore.kernel.org/r/288f26f487648d21fd9590e40b390934eaa5d24a.1711377230.git.christophe.leroy@xxxxxxxxxx > > > > Where it has: > > > > --- a/arch/powerpc/platforms/Kconfig.cputype > > +++ b/arch/powerpc/platforms/Kconfig.cputype > > @@ -98,6 +98,7 @@ config PPC_BOOK3S_64 > > select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION > > select ARCH_ENABLE_SPLIT_PMD_PTLOCK > > select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE > > + select ARCH_HAS_HUGEPD if HUGETLB_PAGE > > select ARCH_SUPPORTS_HUGETLBFS > > select ARCH_SUPPORTS_NUMA_BALANCING > > select HAVE_MOVE_PMD > > @@ -290,6 +291,7 @@ config PPC_BOOK3S > > config PPC_E500 > > select FSL_EMB_PERFMON > > bool > > + select ARCH_HAS_HUGEPD if HUGETLB_PAGE > > select ARCH_SUPPORTS_HUGETLBFS if PHYS_64BIT || PPC64 > > select PPC_SMP_MUXED_IPI > > select PPC_DOORBELL > > > > So I think it means we have three PowerPC systems that supports hugepd > > right now (besides the 8xx which Christophe is trying to drop support > > there), besides 8xx we still have book3s_64 and E500. > > > > Let's check one by one: > > > > - book3s_64 > > > > - hash > > > > - 64K: p4d is not used, largest pgsize pgd 16G @pud level. It > > means after squashing it'll be a bunch of cont-pmd, all good. > > > > - 4K: p4d also not used, largest pgsize pgd 128G, after squashed > > it'll be cont-pud. all good. > > > > - radix > > > > - 64K: largest 1G @pud, then cont-pmd after squashed. all good. > > > > - 4K: largest 1G @pud, then cont-pmd, all good. > > > > - e500 & 8xx > > > > - both of them use 2-level pgtables (pgd + pte), after squashed hugepd > > @pgd level they become cont-pte. all good. > > e500 has two modes: 32 bits and 64 bits. > > For 32 bits: > > 8xx is the only one handling it through HW-assisted pagetable walk hence > requiring a 2-level whatever the pagesize is. Hmm I think maybe finally I get it.. I think the confusion came from when I saw there's always such level-2 table described in Figure 8-5 of the manual: https://www.nxp.com/docs/en/reference-manual/MPC860UM.pdf So I suppose you meant for 8M, the PowerPC 8xx system hardware will be aware of such 8M pgtable (from level-1's entry, where it has bit 28-29 set 011b), then it won't ever read anything starting from "Level-2 Descriptor 1" (but only read the only entry "Level-2 Descriptor 0"), so fundamentally hugepd format must look like such for 8xx? But then perhaps it's still compatible with cont-pte because the rest entries (pte index 1+) will simply be ignored by the hardware? > > On e500 it is all software so pages 2M and larger should be cont-PGD (by > the way I'm a bit puzzled that on arches that have only 2 levels, ie PGD > and PTE, the PGD entries are populated by a function called PMD_populate()). Yeah.. I am also wondering whether pgd_populate() could also work there (perhaps with some trivial changes, or maybe not even needed..), as when p4d/pud/pmd levels are missing, linux should just do something like an enforced cast from pgd_t* -> pmd_t* in this case. I think currently they're already not pgd, as __find_linux_pte() already skipped pgd unconditionally: pgdp = pgdir + pgd_index(ea); p4dp = p4d_offset(pgdp, ea); > > Current situation for 8xx is illustrated here: > https://github.com/linuxppc/wiki/wiki/Huge-pages#8xx > > I also tried to better illustrate e500/32 here: > https://github.com/linuxppc/wiki/wiki/Huge-pages#e500 > > For 64 bits: > We have PTE/PMD/PUD/PGD, no P4D > > See arch/powerpc/include/asm/nohash/64/pgtable-4k.h We don't have anything that is above pud in this category, right? That's what I read from your wiki (and thanks for providing that in the first place; helps a lot for me to understand how it works on PowerPC). I want to make sure if I can move on without caring on p4d/pgd leafs like what we do right now, even after if we can remove hugepd for good, in this case since p4d always missing, then it's about whether "pud|pmd|pte_leaf()" can also cover the pgd ones when that day comes, iiuc. Thanks, > > > > > > I think the trick here is there'll be no pgd leaves after hugepd squashing > > to lower levels, then since PowerPC seems to never have p4d, then all > > things fall into pud or lower. We seem to be all good there? > > > >> > >> If the goal is to purge hugepd then some of the options might turn out > >> to convert hugepd into huge p4d/pgd, as I understand it. It would be > >> nice to have certainty on this at least. > > > > Right. I hope the pmd/pud plan I proposed above can already work too with > > such ambicious goal too. But review very welcomed from either you or > > Christophe. > > > > PS: I think I'll also have a closer look at Christophe's series this week > > or next. > > > >> > >> We have effectively three APIs to parse a single page table and > >> currently none of the APIs can return 100% of the data for power. > > > > Thanks, > > -- Peter Xu
