Re: [PATCH v9 02/14] mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG

[Barry Song <21cnbao@gmail.com>]

On Wed, Mar 9, 2022 at 3:47 PM Yu Zhao <yuzhao@google.com> wrote:
>
> Some architectures support the accessed bit in non-leaf PMD entries,
> e.g., x86 sets the accessed bit in a non-leaf PMD entry when using it
> as part of linear address translation [1]. Page table walkers that
> clear the accessed bit may use this capability to reduce their search
> space.
>
> Note that:
> 1. Although an inline function is preferable, this capability is added
>    as a configuration option for consistency with the existing macros.
> 2. Due to the little interest in other varieties, this capability was
>    only tested on Intel and AMD CPUs.
>
> [1]: Intel 64 and IA-32 Architectures Software Developer's Manual
>      Volume 3 (June 2021), section 4.8
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Acked-by: Brian Geffon <bgeffon@google.com>
> Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
> Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> Acked-by: Steven Barrett <steven@liquorix.net>
> Acked-by: Suleiman Souhlal <suleiman@google.com>
> Tested-by: Daniel Byrne <djbyrne@mtu.edu>
> Tested-by: Donald Carr <d@chaos-reins.com>
> Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
> Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> Tested-by: Shuang Zhai <szhai2@cs.rochester.edu>
> Tested-by: Sofia Trinh <sofia.trinh@edi.works>
> Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> ---

Reviewed-by: Barry Song <baohua@kernel.org>

hard to read this patch by itself. but after reading the change in
walk_pmd_range(), it seems this patch becomes quite clear:
walk_pmd_range()
{
...
#ifdef CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG
                if (get_cap(LRU_GEN_NONLEAF_YOUNG)) {
                        if (!pmd_young(val))
                                continue;

                        walk_pmd_range_locked(pud, addr, vma, walk, &pos);
                }
#endif
...
}
this gives us the chance to skip the scan of all ptes within the
pmd.
so i am not quite sure this should necessarily be a separate
patch, or should be put together with the change in
walk_pmd_range() to make readers understand its purpose.


>  arch/Kconfig                   | 9 +++++++++
>  arch/x86/Kconfig               | 1 +
>  arch/x86/include/asm/pgtable.h | 3 ++-
>  arch/x86/mm/pgtable.c          | 5 ++++-
>  include/linux/pgtable.h        | 4 ++--
>  5 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 678a80713b21..f9c59ecadbbb 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1322,6 +1322,15 @@ config DYNAMIC_SIGFRAME
>  config HAVE_ARCH_NODE_DEV_GROUP
>         bool
>
> +config ARCH_HAS_NONLEAF_PMD_YOUNG
> +       bool
> +       depends on PGTABLE_LEVELS > 2
> +       help
> +         Architectures that select this option are capable of setting the
> +         accessed bit in non-leaf PMD entries when using them as part of linear
> +         address translations. Page table walkers that clear the accessed bit
> +         may use this capability to reduce their search space.
> +
>  source "kernel/gcov/Kconfig"
>
>  source "scripts/gcc-plugins/Kconfig"
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 9f5bd41bf660..e787b7fc75be 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -85,6 +85,7 @@ config X86
>         select ARCH_HAS_PMEM_API                if X86_64
>         select ARCH_HAS_PTE_DEVMAP              if X86_64
>         select ARCH_HAS_PTE_SPECIAL
> +       select ARCH_HAS_NONLEAF_PMD_YOUNG
>         select ARCH_HAS_UACCESS_FLUSHCACHE      if X86_64
>         select ARCH_HAS_COPY_MC                 if X86_64
>         select ARCH_HAS_SET_MEMORY
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 60b6ce45c2e3..f973788f6b21 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -819,7 +819,8 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>
>  static inline int pmd_bad(pmd_t pmd)
>  {
> -       return (pmd_flags(pmd) & ~_PAGE_USER) != _KERNPG_TABLE;
> +       return (pmd_flags(pmd) & ~(_PAGE_USER | _PAGE_ACCESSED)) !=
> +              (_KERNPG_TABLE & ~_PAGE_ACCESSED);
>  }
>
>  static inline unsigned long pages_to_mb(unsigned long npg)
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index 3481b35cb4ec..a224193d84bf 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -550,7 +550,7 @@ int ptep_test_and_clear_young(struct vm_area_struct *vma,
>         return ret;
>  }
>
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
>  int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>                               unsigned long addr, pmd_t *pmdp)
>  {
> @@ -562,6 +562,9 @@ int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>
>         return ret;
>  }
> +#endif
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  int pudp_test_and_clear_young(struct vm_area_struct *vma,
>                               unsigned long addr, pud_t *pudp)
>  {
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 79f64dcff07d..743e7fc4afda 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -212,7 +212,7 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>  #endif
>
>  #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
>  static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>                                             unsigned long address,
>                                             pmd_t *pmdp)
> @@ -233,7 +233,7 @@ static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>         BUILD_BUG();
>         return 0;
>  }
> -#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG */
>  #endif
>
>  #ifndef __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
> --
> 2.35.1.616.g0bdcbb4464-goog
>

Thanks
Barry