Re: powerpc/e500: WARNING: at mm/hugetlb.c:4755 hugetlb_add_hstate

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 10.11.25 19:31, Christophe Leroy wrote:
> 
> 
> Le 10/11/2025 à 12:27, David Hildenbrand (Red Hat) a écrit :
>> Thanks for the review!
>>
>>>
>>> So I think what you want instead is:
>>>
>>> diff --git a/arch/powerpc/platforms/Kconfig.cputype
>>> b/arch/powerpc/platforms/Kconfig.cputype
>>> index 7b527d18aa5ee..1f5a1e587740c 100644
>>> --- a/arch/powerpc/platforms/Kconfig.cputype
>>> +++ b/arch/powerpc/platforms/Kconfig.cputype
>>> @@ -276,6 +276,7 @@ config PPC_E500
>>>            select FSL_EMB_PERFMON
>>>            bool
>>>            select ARCH_SUPPORTS_HUGETLBFS if PHYS_64BIT || PPC64
>>> +       select ARCH_HAS_GIGANTIC_PAGE if ARCH_SUPPORTS_HUGETLBFS
>>>            select PPC_SMP_MUXED_IPI
>>>            select PPC_DOORBELL
>>>            select PPC_KUEP
>>>
>>>
>>>
>>>>         select ARCH_HAS_KCOV
>>>>         select ARCH_HAS_KERNEL_FPU_SUPPORT    if PPC64 && PPC_FPU
>>>>         select ARCH_HAS_MEMBARRIER_CALLBACKS
>>>> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/
>>>> platforms/Kconfig.cputype
>>>> index 7b527d18aa5ee..4c321a8ea8965 100644
>>>> --- a/arch/powerpc/platforms/Kconfig.cputype
>>>> +++ b/arch/powerpc/platforms/Kconfig.cputype
>>>> @@ -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
>>>
>>> Should remain I think.
>>>
>>>>         default y
>>>>         help
>>>>           Enable support for the Power ISA 3.0 Radix style MMU. Currently
>>
>>
>> We also have PPC_8xx do a
>>
>>       select ARCH_SUPPORTS_HUGETLBFS
>>
>> And of course !PPC_RADIX_MMU (e.g., PPC_64S_HASH_MMU) through
>> PPC_BOOK3S_64.
>>
>> Are we sure they cannot end up with gigantic folios through hugetlb?
>>
> 
> Yes indeed. My PPC_8xx is OK because I set CONFIG_ARCH_FORCE_MAX_ORDER=9
> (largest hugepage is 8M) but I do get the warning with the default value
> which is 8 (with 16k pages).
> 
> For PPC_64S_HASH_MMU, max page size is 16M, we get no warning with
> CONFIG_ARCH_FORCE_MAX_ORDER=8 which is the default value but get the
> warning with CONFIG_ARCH_FORCE_MAX_ORDER=7

Right, the dependency on CONFIG_ARCH_FORCE_MAX_ORDER is nasty. In the future,
likely the arch should just tell us the biggest possible hugetlb size and we
can then determine this ourselves.

... or we'll simply remove the gigantic vs. !gigantic handling completely and
simply assume that "if there is hugetlb, we might have gigantic folios".

> Should CONFIG_ARCH_HAS_GIGANTIC_PAGE be set unconditionaly as soon as
> hugepages are selected, or should it depend on
> CONFIG_ARCH_FORCE_MAX_ORDER ? What is the cost of selecting
> CONFIG_ARCH_HAS_GIGANTIC_PAGE ?

There is no real cost, we just try to keep the value small so __dump_folio()
can better detect inconsistencies.

To fix it for now, likely the following is good enough (pushed to the
previously mentioned branch):


 From 7abf0f52e59d96533aa8c96194878e9453aa8be0 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Red Hat)" <david@kernel.org>
Date: Thu, 6 Nov 2025 11:31:45 +0100
Subject: [PATCH] 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 fae7d834c43c ("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 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.

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 (possible on arm64 and 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 relevant
critical enough to mark it as a fix.

Fixes: 7b4f21f5e038 ("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>
---
  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
  	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..2646ba7c96a49 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.
+ */
+#define MAX_FOLIO_ORDER		(16 * GIGA / 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)
+
  # TODO: Allow to be enabled without THP
  config ARCH_SUPPORTS_HUGE_PFNMAP
  	def_bool n
-- 
2.51.0



-- 
Cheers

David