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Hildenbrand (Red Hat) wrote: > On 13.11.25 14:01, Lorenzo Stoakes wrote: > > FYI, trivial to fix but a conflict on mm/Kconfig for mm-new: > > Thanks for the review! > > Yeah, this fix will have to obviously go in sooner. And it's easy to > resolve. > > That's why this patch is already in mm/mm-hotfixes-unstable. Ack. > > [...] > > > > > On Wed, Nov 12, 2025 at 03:56:32PM +0100, David Hildenbrand (Red Hat) wrote: > > > In the past, CONFIG_ARCH_HAS_GIGANTIC_PAGE indicated that we support > > > runtime allocation of gigantic hugetlb folios. In the meantime it evolved > > > into a generic way for the architecture to state that it supports > > > gigantic hugetlb folios. > > > > > > In commit fae7d834c43c ("mm: add __dump_folio()") we started using > > > CONFIG_ARCH_HAS_GIGANTIC_PAGE to decide MAX_FOLIO_ORDER: whether we could > > > > Hm strange commit to introduce this :) > > The first commit to be confused about what CONFIG_ARCH_HAS_GIGANTIC_PAGE > actually means (obviously hugetlb, ... :) ), and which sizes are possible... Yeah, sigh, we love to make things confusing :) > > [...] > > > > > > > To fix it, let's make powerpc select CONFIG_ARCH_HAS_GIGANTIC_PAGE with > > > hugetlb on powerpc, and increase the maximum folio size with hugetlb to 16 > > > GiB (possible on arm64 and powerpc). Note that on some powerpc > > > > I guess this is due to 64 KiB base page possibilities. Fun :) > > > > Will this cause powerpc to now support gigantic hugetlb pages when it didn't > > before? > > It's not really related to 64K IIRC, just the way > CONFIG_ARCH_FORCE_MAX_ORDER and other things interact with powerpcs ways of > mapping cont-pmd-like things for hugetlb. Ah OK, as I was thinking if it's base pages we could just keep order the same... if it's somehow possible to get higher sizes even with without then makes sense to specify. Lord... I wonder if we should have a doc somewhere describing all the ins and outs of this? Not that I'm asking my perenially busy co-maintainer to do _even more_ work but maybe an idea for the future :P > > This patch here doesn't change any of that, it just makes us now correctly > detect that gigantic folios are indeed possible. > > > > > > configurations, whether we actually have gigantic pages > > > depends on the setting of CONFIG_ARCH_FORCE_MAX_ORDER, but there is > > > nothing really problematic about setting it unconditionally: we just try to > > > keep the value small so we can better detect problems in __dump_folio() > > > and inconsistencies around the expected largest folio in the system. > > > > > > Ideally, we'd have a better way to obtain the maximum hugetlb folio size > > > and detect ourselves whether we really end up with gigantic folios. Let's > > > defer bigger changes and fix the warnings first. > > > > Right. > > > > > > > > While at it, handle gigantic DAX folios more clearly: DAX can only > > > end up creating gigantic folios with HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD. > > > > Yes, this is... quite something. Config implying gigantic THP possible but > > actually only relevant to DAX... > > > > > > > > Add a new Kconfig option HAVE_GIGANTIC_FOLIOS to make both cases > > > clearer. In particular, worry about ARCH_HAS_GIGANTIC_PAGE only with > > > HUGETLB_PAGE. > > > > Hm, I see: > > > > config HUGETLB_PAGE > > def_bool HUGETLBFS > > select XARRAY_MULTI > > > > > > Which means (unless I misunderstand Kconfig, very possible :) that this is > > always set if HUGETLBFS is specified. > > Yeah, def_bool enforces that both are set. > > > Would it be clearer to just check for > > CONFIG_HUGETLBFS? > > IMHO, MM code should focus on CONFIG_HUGETLB_PAGE (especially when dealing > with the page/folio aspects), not the FS part of it. Yeah this is another weird fs/mm split for something that really is ultimately an mm thing... > > $ git grep CONFIG_HUGETLB_PAGE | wc -l > 45 > $ git grep CONFIG_HUGETLBFS | wc -l > 7 > > Unsurprisingly, we are not being completely consistent :) Well fair enough :) > > > > > > > > > Note: with enabling CONFIG_ARCH_HAS_GIGANTIC_PAGE on powerpc, we will now > > > also allow for runtime allocations of folios in some more powerpc configs. > > > > Ah OK you're answering the above. I mean I don't think it'll be a problem > > either. > > > > > I don't think this is a problem, but if it is we could handle it through > > > __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED. > > > > > > While __dump_page()/__dump_folio was also problematic (not handling dumping > > > of tail pages of such gigantic folios correctly), it doesn't relevant > > > critical enough to mark it as a fix. > > > > Small typo 'it doesn't relevant critical enough' -> 'it doesn't seem > > critical enough' perhaps? Doesn't really matter, only fixup if respin or > > easy for Andrew to fix. > > Ah yes, thanks. > > > > > Are you planning to do follow ups then I guess? > > As time permits, I think this all needs to be reworked :( Yup! :) > > [...] > > > > @@ -137,6 +137,7 @@ config PPC > > > select ARCH_HAS_DMA_OPS if PPC64 > > > select ARCH_HAS_FORTIFY_SOURCE > > > select ARCH_HAS_GCOV_PROFILE_ALL > > > + select ARCH_HAS_GIGANTIC_PAGE if ARCH_SUPPORTS_HUGETLBFS > > > > Given we know the architecture can support it (presumably all powerpc > > arches or all that can support hugetlbfs anyway?), this seems reasonable. > > powerpc allows for quite some different configs, so I assume there are some > configs that don't allow ARCH_SUPPORTS_HUGETLBFS. Ah OK. > > [...] > > > > /* > > > * There is no real limit on the folio size. We limit them to the maximum we > > > - * currently expect (e.g., hugetlb, dax). > > > + * currently expect: with hugetlb, we expect no folios larger than 16 GiB. > > > > Maybe worth saying 'see CONFIG_HAVE_GIGANTIC_FOLIOS definition' or something? > > To me that's implied from the initial ifdef. But not strong opinion about > spelling that out. > > > > > > + */ > > > +#define MAX_FOLIO_ORDER get_order(SZ_16G) > > > > Hmm, is the base page size somehow runtime adjustable on powerpc? Why isn't > > PUD_ORDER good enough here? > > We tried P4D_ORDER but even that doesn't work. I think we effectively end up > with cont-pmd/cont-PUD mappings (or even cont-p4d, I am not 100% sure > because the folding code complicates that). Ah wow, didn't even know such things could be a thing :) > > See powerpcs variant of huge_pte_alloc() where we have stuff like > > p4d = p4d_offset(pgd_offset(mm, addr), addr); > if (!mm_pud_folded(mm) && sz >= P4D_SIZE) > return (pte_t *)p4d; > > As soon as we go to things like P4D_ORDER we're suddenly in the range of 512 > GiB on x86 etc, so that's also not what we want as an easy fix. (and it > didn't work) Yeah... better to be explicit about the ppc case I think you're right. > > > > > Or does powerpc have some way of getting 16 GiB gigantic pages even with 4 > > KiB base page size? > > IIUC, yes. > > Take a look at MMU_PAGE_16G. Ack yeah, surprising, but these arches can be a whole other world... too used to basic arm64/x86-64 :) > > There is MMU_PAGE_64G already defined, but it's essentially unused for now. Hmm :) > > > > > > +#else > > > +/* > > > + * Without hugetlb, gigantic folios that are bigger than a single PUD are > > > + * currently impossible. > > > */ > > > #define MAX_FOLIO_ORDER PUD_ORDER > > > #endif > > > diff --git a/mm/Kconfig b/mm/Kconfig > > > index 0e26f4fc8717b..ca3f146bc7053 100644 > > > --- a/mm/Kconfig > > > +++ b/mm/Kconfig > > > @@ -908,6 +908,13 @@ config PAGE_MAPCOUNT > > > config PGTABLE_HAS_HUGE_LEAVES > > > def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE > > > > > > +# > > > +# We can end up creating gigantic folio. > > > +# > > > +config HAVE_GIGANTIC_FOLIOS > > > + def_bool (HUGETLB_PAGE && ARCH_HAS_GIGANTIC_PAGE) || \ > > > + (ZONE_DEVICE && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) > > > > Maybe worth spelling out in a comment these two cases? > > Not sure if the comments wouldn't just explain what we are reading? > > "gigantic folios with hugetlb, PUD-sized folios with ZONE_DEVICE"? Yeah true not vital. > > -- > Cheers > > David Cheers, Lorenzo