Re: [PATCH v2] arm64: enable THP_SWAP for arm64

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 27 May 2022 22:06:44 +1200 Barry Song <21cnbao@gmail.com> wrote:

> From: Barry Song <v-songbaohua@oppo.com>
> 
> THP_SWAP has been proved to improve the swap throughput significantly
> on x86_64 according to commit bd4c82c22c367e ("mm, THP, swap: delay
> splitting THP after swapped out").
> As long as arm64 uses 4K page size, it is quite similar with x86_64
> by having 2MB PMD THP. So we are going to get similar improvement.
> For other page sizes such as 16KB and 64KB, PMD might be too large.
> Negative side effects such as IO latency might be a problem. Thus,
> we can only safely enable the counterpart of X86_64.
> A corner case is that MTE has an assumption that only base pages
> can be swapped. We won't enable THP_SWP for ARM64 hardware with
> MTE support until MTE is re-arched.
> 
> ...
>
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -45,6 +45,8 @@
>  	__flush_tlb_range(vma, addr, end, PUD_SIZE, false, 1)
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> +#define arch_thp_swp_supported !system_supports_mte

Does that even work?

	if (arch_thp_swp_supported())

expands to

	if (!system_supports_mte())

so I guess it does work.  Is this ugly party trick required for some
reason?  If so, an apologetic comment describing why would be helpful.

Otherwise, can we use a static inline function here, as we do with the
stub function?

> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -461,4 +461,16 @@ static inline int split_folio_to_list(struct folio *folio,
>  	return split_huge_page_to_list(&folio->page, list);
>  }
>  
> +/*
> + * archs that select ARCH_WANTS_THP_SWAP but don't support THP_SWP due to
> + * limitations in the implementation like arm64 MTE can override this to
> + * false
> + */
> +#ifndef arch_thp_swp_supported
> +static inline bool arch_thp_swp_supported(void)
> +{
> +	return true;
> +}

Missing a #define arch_thp_swp_supported arch_thp_swp_supported here.

> +#endif
> +
>  #endif /* _LINUX_HUGE_MM_H */

Otherwise looks OK to me.  Please include it in the arm64 tree if/when
it's considered ready.