Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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
