linux-kernel - Re: [PATCH v15 3/4] mm: define default MAX_PTRS_PER

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Date:   Sun, 20 Jun 2021 14:17:04 +0300
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Daniel Axtens <dja@...ens.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, christophe.leroy@...roup.eu,
        aneesh.kumar@...ux.ibm.com, bsingharora@...il.com
Subject: Re: [PATCH v15 3/4] mm: define default MAX_PTRS_PER_* in include/pgtable.h

On Thu, Jun 17, 2021 at 12:30 PM Daniel Axtens <dja@...ens.net> wrote:
>
> 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.
>
> Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>
> Reviewed-by: Marco Elver <elver@...gle.com>
> Signed-off-by: Daniel Axtens <dja@...ens.net>
>
> ---
>
> 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 */
> --
> 2.30.2
>

Acked-by: Andrey Konovalov <andreyknvl@...il.com>