linux-kernel - Re: [PATCH v2] mm: fix MAX_FOLIO_ORDER on powerpc configs with hugetlb

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Message-ID: <8092114e-f6f3-481a-966d-5b8289859a46@linux.ibm.com>
Date: Mon, 17 Nov 2025 09:19:24 +0530
From: Madhavan Srinivasan <maddy@...ux.ibm.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
        "David Hildenbrand (Red Hat)" <david@...nel.org>,
        linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        Donet Tom <donettom@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.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>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2] mm: fix MAX_FOLIO_ORDER on powerpc configs with
 hugetlb



On 11/15/25 3:07 PM, Christophe Leroy wrote:
> 
> 
> Le 14/11/2025 à 22:49, David Hildenbrand (Red Hat) a écrit :
>> 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 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.
> 
> Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>
> 
> Tested on powerpc 8xx with CONFIG_ARCH_FORCE_MAX_ORDER=8 instead of 9.
> It is now possible to add hugepages with the following command:
> 
> echo 4 > /sys/kernel/mm/hugepages/hugepages-8192kB/nr_hugepages
> 
> But only if CONFIG_CMA is set.
> 
> Tested-by: Christophe Leroy <christophe.leroy@...roup.eu>
> 

Acked-by: Madhavan Srinivasan <maddy@...ux.ibm.com>

Andrew,
can you please take this fix via mm tree

Maddy

>>
>> 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.
>>
>> 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>
>> Cc: Nathan Chancellor <nathan@...nel.org>
>> Signed-off-by: David Hildenbrand (Red Hat) <david@...nel.org>
>> ---
>>
>> v1 -> v2:
>> * Adjust patch description (typo, 16G vs 1G)
>> * Remove ARCH_HAS_GIGANTIC_PAGE from arch/powerpc/platforms/Kconfig.cputype
>> * Mention CONFIG_HAVE_GIGANTIC_FOLIOS in comment
>> * Use 1 GiB on 32bit to avoid unsigned-long capacity issues
>>
>> I yet have to boot-test this on 32bit powerpc. Something for Monday.
>>
>> ---
>>   arch/powerpc/Kconfig                   |  1 +
>>   arch/powerpc/platforms/Kconfig.cputype |  1 -
>>   include/linux/mm.h                     | 13 ++++++++++---
>>   mm/Kconfig                             |  7 +++++++
>>   4 files changed, 18 insertions(+), 4 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/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
>>       default y
>>       help
>>         Enable support for the Power ISA 3.0 Radix style MMU. Currently this
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index d16b33bacc32b..7c79b3369b82c 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 +2087,17 @@ 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
>> 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
>>
>> base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
>