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Date: Tue, 2 Apr 2024 12:05:49 -0700
From: Nathan Chancellor <nathan@kernel.org>
To: peterx@redhat.com
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Yang Shi <shy828301@gmail.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
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	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>, loongarch@lists.linux.dev
Subject: Re: [PATCH v4 05/13] mm/arch: Provide pud_pfn() fallback
Message-ID: <20240402190549.GA706730@dev-arch.thelio-3990X>
References: <20240327152332.950956-1-peterx@redhat.com>
 <20240327152332.950956-6-peterx@redhat.com>
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In-Reply-To: <20240327152332.950956-6-peterx@redhat.com>

Hi Peter (and LoongArch folks),

On Wed, Mar 27, 2024 at 11:23:24AM -0400, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> The comment in the code explains the reasons.  We took a different approach
> comparing to pmd_pfn() by providing a fallback function.
> 
> Another option is to provide some lower level config options (compare to
> HUGETLB_PAGE or THP) to identify which layer an arch can support for such
> huge mappings.  However that can be an overkill.
> 
> Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  arch/riscv/include/asm/pgtable.h    |  1 +
>  arch/s390/include/asm/pgtable.h     |  1 +
>  arch/sparc/include/asm/pgtable_64.h |  1 +
>  arch/x86/include/asm/pgtable.h      |  1 +
>  include/linux/pgtable.h             | 10 ++++++++++
>  5 files changed, 14 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 20242402fc11..0ca28cc8e3fa 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -646,6 +646,7 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
>  
>  #define __pud_to_phys(pud)  (__page_val_to_pfn(pud_val(pud)) << PAGE_SHIFT)
>  
> +#define pud_pfn pud_pfn
>  static inline unsigned long pud_pfn(pud_t pud)
>  {
>  	return ((__pud_to_phys(pud) & PUD_MASK) >> PAGE_SHIFT);
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 1a71cb19c089..6cbbe473f680 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1414,6 +1414,7 @@ static inline unsigned long pud_deref(pud_t pud)
>  	return (unsigned long)__va(pud_val(pud) & origin_mask);
>  }
>  
> +#define pud_pfn pud_pfn
>  static inline unsigned long pud_pfn(pud_t pud)
>  {
>  	return __pa(pud_deref(pud)) >> PAGE_SHIFT;
> diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> index 4d1bafaba942..26efc9bb644a 100644
> --- a/arch/sparc/include/asm/pgtable_64.h
> +++ b/arch/sparc/include/asm/pgtable_64.h
> @@ -875,6 +875,7 @@ static inline bool pud_leaf(pud_t pud)
>  	return pte_val(pte) & _PAGE_PMD_HUGE;
>  }
>  
> +#define pud_pfn pud_pfn
>  static inline unsigned long pud_pfn(pud_t pud)
>  {
>  	pte_t pte = __pte(pud_val(pud));
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index cefc7a84f7a4..273f7557218c 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -234,6 +234,7 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
>  	return (pfn & pmd_pfn_mask(pmd)) >> PAGE_SHIFT;
>  }
>  
> +#define pud_pfn pud_pfn
>  static inline unsigned long pud_pfn(pud_t pud)
>  {
>  	phys_addr_t pfn = pud_val(pud);
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 600e17d03659..75fe309a4e10 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1817,6 +1817,16 @@ typedef unsigned int pgtbl_mod_mask;
>  #define pte_leaf_size(x) PAGE_SIZE
>  #endif
>  
> +/*
> + * We always define pmd_pfn for all archs as it's used in lots of generic
> + * code.  Now it happens too for pud_pfn (and can happen for larger
> + * mappings too in the future; we're not there yet).  Instead of defining
> + * it for all archs (like pmd_pfn), provide a fallback.
> + */
> +#ifndef pud_pfn
> +#define pud_pfn(x) ({ BUILD_BUG(); 0; })
> +#endif
> +
>  /*
>   * Some architectures have MMUs that are configurable or selectable at boot
>   * time. These lead to variable PTRS_PER_x. For statically allocated arrays it
> -- 
> 2.44.0
> 

This BUILD_BUG() triggers for LoongArch with their defconfig, so it
seems like they need to provide an implementation of pud_pfn()?

  In function 'follow_huge_pud',
      inlined from 'follow_pud_mask' at mm/gup.c:1075:10,
      inlined from 'follow_p4d_mask' at mm/gup.c:1105:9,
      inlined from 'follow_page_mask' at mm/gup.c:1151:10:
  include/linux/compiler_types.h:460:45: error: call to '__compiletime_assert_382' declared with attribute error: BUILD_BUG failed
    460 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |                                             ^
  include/linux/compiler_types.h:441:25: note: in definition of macro '__compiletime_assert'
    441 |                         prefix ## suffix();                             \
        |                         ^~~~~~
  include/linux/compiler_types.h:460:9: note: in expansion of macro '_compiletime_assert'
    460 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |         ^~~~~~~~~~~~~~~~~~~
  include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
     39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
        |                                     ^~~~~~~~~~~~~~~~~~
  include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
     59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
        |                     ^~~~~~~~~~~~~~~~
  include/linux/pgtable.h:1887:23: note: in expansion of macro 'BUILD_BUG'
   1887 | #define pud_pfn(x) ({ BUILD_BUG(); 0; })
        |                       ^~~~~~~~~
  mm/gup.c:679:29: note: in expansion of macro 'pud_pfn'
    679 |         unsigned long pfn = pud_pfn(pud);
        |                             ^~~~~~~

Cheers,
Nathan