Message-ID: <eb1df7ba-1ab6-4c0d-a54a-52d642595259@lucifer.local>
Date: Mon, 17 Nov 2025 10:16:09 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Donet Tom <donettom@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v1] mm: fix MAX_FOLIO_ORDER on powerpc configs with
 hugetlb

On Thu, Nov 13, 2025 at 04:21:41PM +0100, David 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

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