Message-ID: <eaf2e733-f967-43bb-88e6-7876a28a370c@lucifer.local>
Date: Thu, 13 Nov 2025 13:01:42 +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

FYI, trivial to fix but a conflict on mm/Kconfig for mm-new:

<<<<<<< HEAD
config ASYNC_KERNEL_PGTABLE_FREE
	def_bool n
=======
#
# 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)
>>>>>>> mm: fix MAX_FOLIO_ORDER on powerpc configs with hugetlb

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 :)

> have folios larger than what the buddy can handle. In the context of
> that commit, we started using MAX_FOLIO_ORDER to detect page corruptions
> when dumping tail pages of folios. Before that commit, we assumed that
> we cannot have folios larger than the highest buddy order, which was
> obviously wrong.
>
> In commit 7b4f21f5e038 ("mm/hugetlb: check for unreasonable folio sizes
> when registering hstate"), we used MAX_FOLIO_ORDER to detect
> inconsistencies, and in fact, we found some now.

Makes sense. And this is what the report bisects the issue to.

>
> Powerpc allows for configs that can allocate gigantic folio during boot
> (not at runtime), that do not set CONFIG_ARCH_HAS_GIGANTIC_PAGE and can
> exceed PUD_ORDER.

God...

>
> 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?

> 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. Would it be clearer to just check for
CONFIG_HUGETLBFS?

>
> 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.

Are you planning to do follow ups then I guess?

>
> Fixes: 7b4f21f5e038 ("mm/hugetlb: check for unreasonable folio sizes when registering hstate")
> Reported-by: Christophe Leroy <christophe.leroy@...roup.eu>
> Closes: https://lore.kernel.org/r/3e043453-3f27-48ad-b987-cc39f523060a@csgroup.eu/
> Reported-by: Sourabh Jain <sourabhjain@...ux.ibm.com>
> Closes: https://lore.kernel.org/r/94377f5c-d4f0-4c0f-b0f6-5bf1cd7305b1@linux.ibm.com/
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
> Cc: Donet Tom <donettom@...ux.ibm.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: David Hildenbrand (Red Hat) <david@...nel.org>
> ---
>  arch/powerpc/Kconfig |  1 +
>  include/linux/mm.h   | 12 +++++++++---
>  mm/Kconfig           |  7 +++++++
>  3 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index e24f4d88885ae..9537a61ebae02 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -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.

>  	select ARCH_HAS_KCOV
>  	select ARCH_HAS_KERNEL_FPU_SUPPORT	if PPC64 && PPC_FPU
>  	select ARCH_HAS_MEMBARRIER_CALLBACKS
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d16b33bacc32b..63aea4b3fb5d9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2074,7 +2074,7 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
>  	return folio_large_nr_pages(folio);
>  }
>
> -#if !defined(CONFIG_ARCH_HAS_GIGANTIC_PAGE)
> +#if !defined(CONFIG_HAVE_GIGANTIC_FOLIOS)
>  /*
>   * We don't expect any folios that exceed buddy sizes (and consequently
>   * memory sections).
> @@ -2087,10 +2087,16 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
>   * pages are guaranteed to be contiguous.
>   */
>  #define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT
> -#else
> +#elif defined(CONFIG_HUGETLB_PAGE)
>  /*
>   * 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?

> + */
> +#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?

Or does powerpc have some way of getting 16 GiB gigantic pages even with 4
KiB base page size?

> +#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?

> +
>  # TODO: Allow to be enabled without THP
>  config ARCH_SUPPORTS_HUGE_PFNMAP
>  	def_bool n
> --
> 2.51.0
>

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