linux-kernel - Re: [PATCH v3 7/7] mm/swap: Cache swap migration A/D bits support

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Date:   Wed, 10 Aug 2022 14:37:40 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 7/7] mm/swap: Cache swap migration A/D bits support

Peter Xu <peterx@...hat.com> writes:

> Introduce a variable swap_migration_ad_supported to cache whether the arch
> supports swap migration A/D bits.
>
> Here one thing to mention is that SWP_MIG_TOTAL_BITS will internally
> reference the other macro MAX_PHYSMEM_BITS, which is a function call on
> x86 (constant on all the rest of archs).
>
> It's safe to reference it in swapfile_init() because when reaching here
> we're already during initcalls level 4 so we must have initialized 5-level
> pgtable for x86_64 (right after early_identify_cpu() finishes).
>
> - start_kernel
>   - setup_arch
>     - early_cpu_init
>       - get_cpu_cap --> fetch from CPUID (including X86_FEATURE_LA57)
>       - early_identify_cpu --> clear X86_FEATURE_LA57 (if early lvl5 not enabled (USE_EARLY_PGTABLE_L5))
>   - arch_call_rest_init
>     - rest_init
>       - kernel_init
>         - kernel_init_freeable
>           - do_basic_setup
>             - do_initcalls --> calls swapfile_init() (initcall level 4)
>
> This should slightly speed up the migration swap entry handlings.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  include/linux/swapfile.h | 1 +
>  include/linux/swapops.h  | 7 +------
>  mm/swapfile.c            | 8 ++++++++
>  3 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
> index 54078542134c..87ec5e2cdb02 100644
> --- a/include/linux/swapfile.h
> +++ b/include/linux/swapfile.h
> @@ -9,5 +9,6 @@
>  extern struct swap_info_struct *swap_info[];
>  extern unsigned long generic_max_swapfile_size(void);
>  extern unsigned long max_swapfile_size(void);
> +extern bool swap_migration_ad_supported;
>  
>  #endif /* _LINUX_SWAPFILE_H */
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 0e9579b90659..e6afc77c51ad 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -301,13 +301,8 @@ static inline swp_entry_t make_writable_migration_entry(pgoff_t offset)
>   */
>  static inline bool migration_entry_supports_ad(void)
>  {
> -	/*
> -	 * max_swapfile_size() returns the max supported swp-offset plus 1.
> -	 * We can support the migration A/D bits iff the pfn swap entry has
> -	 * the offset large enough to cover all of them (PFN, A & D bits).
> -	 */
>  #ifdef CONFIG_SWAP
> -	return max_swapfile_size() >= (1UL << SWP_MIG_TOTAL_BITS);
> +	return swap_migration_ad_supported;
>  #else  /* CONFIG_SWAP */
>  	return false;
>  #endif	/* CONFIG_SWAP */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 794fa37bd0c3..c49cf25f0d08 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -64,6 +64,9 @@ EXPORT_SYMBOL_GPL(nr_swap_pages);
>  long total_swap_pages;
>  static int least_priority = -1;
>  static unsigned long swapfile_maximum_size;
> +#ifdef CONFIG_MIGRATION
> +bool swap_migration_ad_supported;
> +#endif	/* CONFIG_MIGRATION */
>  
>  static const char Bad_file[] = "Bad swap file entry ";
>  static const char Unused_file[] = "Unused swap file entry ";
> @@ -3685,6 +3688,11 @@ static int __init swapfile_init(void)
>  
>  	swapfile_maximum_size = arch_max_swapfile_size();
>  
> +#ifdef CONFIG_MIGRATION
> +	if (swapfile_maximum_size >= (1UL << SWP_MIG_TOTAL_BITS))
> +		swap_migration_ad_supported = true;
> +#endif	/* CONFIG_MIGRATION */
> +
>  	return 0;
>  }
>  subsys_initcall(swapfile_init);

I don't think it's necessary to add a variable for such a simple
function and it's not a super hot path.  But I don't have strong
opinions here.

Best Regards,
Huang, Ying