Re: [PATCH v1] mm: fix MAX_FOLIO_ORDER on powerpc configs with hugetlb

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

On 12.11.25 15:56, 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
> 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/
> Cc: Andrew Morton <akpm@linux-foundation.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>
> 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..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.
> + */
> +#define MAX_FOLIO_ORDER		get_order(SZ_16G)

Turns out that's a problem on 32bit builds, because it won't fit into unsigned long. Grml.

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 63aea4b3fb5d9..f595565bdd113 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2090,9 +2090,10 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
  #elif defined(CONFIG_HUGETLB_PAGE)
  /*
   * There is no real limit on the folio size. We limit them to the maximum we
- * currently expect: with hugetlb, we expect no folios larger than 16 GiB.
+ * currently expect: with hugetlb, we expect no folios larger than 16 GiB
+ * on 64bit and 1 GiB on 32bit.
   */
-#define MAX_FOLIO_ORDER                get_order(SZ_16G)
+#define MAX_FOLIO_ORDER                get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
  #else
  /*

I'll resend the patch ...


-- 
Cheers

David