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Subject: Re: [PATCH v14 3/4] mm: define default MAX_PTRS_PER_* in
 include/pgtable.h
To:     Daniel Axtens <dja@axtens.net>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, kasan-dev@googlegroups.com, elver@google.com,
        akpm@linux-foundation.org, andreyknvl@gmail.com
Cc:     linuxppc-dev@lists.ozlabs.org, aneesh.kumar@linux.ibm.com,
        bsingharora@gmail.com
References: <20210617063956.94061-1-dja@axtens.net>
 <20210617063956.94061-4-dja@axtens.net>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <cbe77a1d-074d-4bc0-0aad-996249a6bf3a@csgroup.eu>
Date:   Thu, 17 Jun 2021 09:00:35 +0200
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Le 17/06/2021 à 08:39, Daniel Axtens a écrit :
> Commit c65e774fb3f6 ("x86/mm: Make PGDIR_SHIFT and PTRS_PER_P4D variable")
> made PTRS_PER_P4D variable on x86 and introduced MAX_PTRS_PER_P4D as a
> constant for cases which need a compile-time constant (e.g. fixed-size
> arrays).
> 
> powerpc likewise has boot-time selectable MMU features which can cause
> other mm "constants" to vary. For KASAN, we have some static
> PTE/PMD/PUD/P4D arrays so we need compile-time maximums for all these
> constants. Extend the MAX_PTRS_PER_ idiom, and place default definitions
> in include/pgtable.h. These define MAX_PTRS_PER_x to be PTRS_PER_x unless
> an architecture has defined MAX_PTRS_PER_x in its arch headers.
> 
> Clean up pgtable-nop4d.h and s390's MAX_PTRS_PER_P4D definitions while
> we're at it: both can just pick up the default now.
> 
> Signed-off-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> 
> ---
> 
> s390 was compile tested only.
> ---
>   arch/s390/include/asm/pgtable.h     |  2 --
>   include/asm-generic/pgtable-nop4d.h |  1 -
>   include/linux/pgtable.h             | 22 ++++++++++++++++++++++
>   3 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 7c66ae5d7e32..cf05954ce013 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -342,8 +342,6 @@ static inline int is_module_addr(void *addr)
>   #define PTRS_PER_P4D	_CRST_ENTRIES
>   #define PTRS_PER_PGD	_CRST_ENTRIES
>   
> -#define MAX_PTRS_PER_P4D	PTRS_PER_P4D
> -
>   /*
>    * Segment table and region3 table entry encoding
>    * (R = read-only, I = invalid, y = young bit):
> diff --git a/include/asm-generic/pgtable-nop4d.h b/include/asm-generic/pgtable-nop4d.h
> index ce2cbb3c380f..2f6b1befb129 100644
> --- a/include/asm-generic/pgtable-nop4d.h
> +++ b/include/asm-generic/pgtable-nop4d.h
> @@ -9,7 +9,6 @@
>   typedef struct { pgd_t pgd; } p4d_t;
>   
>   #define P4D_SHIFT		PGDIR_SHIFT
> -#define MAX_PTRS_PER_P4D	1
>   #define PTRS_PER_P4D		1
>   #define P4D_SIZE		(1UL << P4D_SHIFT)
>   #define P4D_MASK		(~(P4D_SIZE-1))
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 9e6f71265f72..69700e3e615f 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1625,4 +1625,26 @@ typedef unsigned int pgtbl_mod_mask;
>   #define pte_leaf_size(x) PAGE_SIZE
>   #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
> + * helps to have a static maximum value.
> + */
> +
> +#ifndef MAX_PTRS_PER_PTE
> +#define MAX_PTRS_PER_PTE PTRS_PER_PTE
> +#endif
> +
> +#ifndef MAX_PTRS_PER_PMD
> +#define MAX_PTRS_PER_PMD PTRS_PER_PMD
> +#endif
> +
> +#ifndef MAX_PTRS_PER_PUD
> +#define MAX_PTRS_PER_PUD PTRS_PER_PUD
> +#endif
> +
> +#ifndef MAX_PTRS_PER_P4D
> +#define MAX_PTRS_PER_P4D PTRS_PER_P4D
> +#endif
> +
>   #endif /* _LINUX_PGTABLE_H */
>