Commit 39231e8d authored Nov 14, 2025 by David Hildenbrand (Red Hat) Committed by Andrew Morton Nov 15, 2025

mm: fix MAX_FOLIO_ORDER on powerpc configs with hugetlb

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 fae7d834 ("mm: add __dump_folio()") we started using
CONFIG_ARCH_HAS_GIGANTIC_PAGE to decide MAX_FOLIO_ORDER: whether we could
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 7b4f21f5 ("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.

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.

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 on 64bit (possible on arm64 and powerpc) and 1 GiB on 32 bit
(powerpc).  Note that on some powerpc 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.

While at it, handle gigantic DAX folios more clearly: DAX can only end up
creating gigantic folios with HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD.

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.

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.
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 seem
critical enough to mark it as a fix.

Link: https://lkml.kernel.org/r/20251114214920.2550676-1-david@kernel.org


Fixes: 7b4f21f5 ("mm/hugetlb: check for unreasonable folio sizes when registering hstate")
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Closes: https://lore.kernel.org/r/3e043453-3f27-48ad-b987-cc39f523060a@csgroup.eu/


Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Closes: https://lore.kernel.org/r/94377f5c-d4f0-4c0f-b0f6-5bf1cd7305b1@linux.ibm.com/


Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent b05addf6

arch/powerpc/Kconfig

+1 −0

Original line number	Diff line number	Diff line
		@@ -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
		select ARCH_HAS_KCOV
		select ARCH_HAS_KERNEL_FPU_SUPPORT if PPC64 && PPC_FPU
		select ARCH_HAS_MEMBARRIER_CALLBACKS

arch/powerpc/platforms/Kconfig.cputype

+0 −1

Original line number	Diff line number	Diff line
		@@ -423,7 +423,6 @@ config PPC_64S_HASH_MMU
		config PPC_RADIX_MMU
		bool "Radix MMU Support"
		depends on PPC_BOOK3S_64
		select ARCH_HAS_GIGANTIC_PAGE
		default y
		help
		Enable support for the Power ISA 3.0 Radix style MMU. Currently this

include/linux/mm.h

+10 −3

Original line number	Diff line number	Diff line
		@@ -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 +2087,17 @@ 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 (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
		* no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
		*/
		#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
		#else
		/*
		* Without hugetlb, gigantic folios that are bigger than a single PUD are
		* currently impossible.
		*/
		#define MAX_FOLIO_ORDER PUD_ORDER
		#endif

mm/Kconfig

+7 −0

Original line number	Diff line number	Diff line
		@@ -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)

		# TODO: Allow to be enabled without THP
		config ARCH_SUPPORTS_HUGE_PFNMAP
		def_bool n