Re: [RESEND PATCH v3] arm64: enable THP_SWAP for arm64

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

On Tue, Jul 19, 2022 at 4:06 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Jul 18, 2022 at 09:00:50PM +1200, Barry Song wrote:
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 1652a9800ebe..e1c540e80eec 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -101,6 +101,7 @@ config ARM64
> >       select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> >       select ARCH_WANT_LD_ORPHAN_WARN
> >       select ARCH_WANTS_NO_INSTR
> > +     select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES
>
> Can't you avoid all the other changes by simply doing:
>
>         select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES && !ARM64_MTE
>
> >       select ARCH_HAS_UBSAN_SANITIZE_ALL
> >       select ARM_AMBA
> >       select ARM_ARCH_TIMER

Nope. as we also enable ARM64_MTE on platforms without mte.
ARMv8.5 based processors introduce the Memory Tagging Extension
(MTE) feature but the Kconfig is default Y for platforms before 8.5.

arm64 usually detects the cpufeature rather than depending on
static build configuration.

> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 0b6632f18364..78d6f6014bfb 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -45,6 +45,12 @@
> >       __flush_tlb_range(vma, addr, end, PUD_SIZE, false, 1)
> >  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> > +static inline bool arch_thp_swp_supported(void)
> > +{
> > +     return !system_supports_mte();
> > +}
> > +#define arch_thp_swp_supported arch_thp_swp_supported
> > +
> >  /*
> >   * Outside of a few very special situations (e.g. hibernation), we always
> >   * use broadcast TLB invalidation instructions, therefore a spurious page
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index de29821231c9..4ddaf6ad73ef 100644
> > --- 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;
> > +}
> > +#endif
> > +
> >  #endif /* _LINUX_HUGE_MM_H */
> > diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> > index 2a65a89b5b4d..10b94d64cc25 100644
> > --- a/mm/swap_slots.c
> > +++ b/mm/swap_slots.c
> > @@ -307,7 +307,7 @@ swp_entry_t folio_alloc_swap(struct folio *folio)
> >       entry.val = 0;
> >
> >       if (folio_test_large(folio)) {
> > -             if (IS_ENABLED(CONFIG_THP_SWAP))
> > +             if (IS_ENABLED(CONFIG_THP_SWAP) && arch_thp_swp_supported())
> >                       get_swap_pages(1, &entry, folio_nr_pages(folio));
> >               goto out;
> >       }
> > --
> > 2.25.1
> >
> >

Thanks
Barry