Re: [PATCH] arm64: enable THP_SWAP for arm64

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, May 24, 2022 at 10:05:35PM +1200, Barry Song wrote:
> On Tue, May 24, 2022 at 8:12 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Tue, May 24, 2022 at 07:14:03PM +1200, Barry Song wrote:
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index d550f5acfaf3..8e3771c56fbf 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -98,6 +98,7 @@ config ARM64
> > >       select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36)
> > >       select ARCH_WANT_LD_ORPHAN_WARN
> > >       select ARCH_WANTS_NO_INSTR
> > > +     select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES
> >
> > I'm not opposed to this but I think it would break pages mapped with
> > PROT_MTE. We have an assumption in mte_sync_tags() that compound pages
> > are not swapped out (or in). With MTE, we store the tags in a slab
> 
> I assume you mean mte_sync_tags() require that THP is not swapped as a whole,
> as without THP_SWP, THP is still swapping after being splitted. MTE doesn't stop
> THP from swapping through a couple of splitted pages, does it?

That's correct, split THP page are swapped out/in just fine.

> > object (128-bytes per swapped page) and restore them when pages are
> > swapped in. At some point we may teach the core swap code about such
> > metadata but in the meantime that was the easiest way.
> 
> If my previous assumption is true,  the easiest way to enable THP_SWP
> for this moment might be always letting mm fallback to the splitting
> way for MTE hardware. For this moment, I care about THP_SWP more as
> none of my hardware has MTE.
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 45c358538f13..d55a2a3e41a9 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -44,6 +44,8 @@
>         __flush_tlb_range(vma, addr, end, PUD_SIZE, false, 1)
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> 
> +#define arch_thp_swp_supported !system_supports_mte
> +
>  /*
>   * 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 2999190adc22..064b6b03df9e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -447,4 +447,16 @@ 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 2b5531840583..dde685836328 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -309,7 +309,7 @@ swp_entry_t get_swap_page(struct page *page)
>         entry.val = 0;
> 
>         if (PageTransHuge(page)) {
> -               if (IS_ENABLED(CONFIG_THP_SWAP))
> +               if (IS_ENABLED(CONFIG_THP_SWAP) && arch_thp_swp_supported())
>                         get_swap_pages(1, &entry, HPAGE_PMD_NR);
>                 goto out;

I think this should work and with your other proposal it would be
limited to MTE pages:

#define arch_thp_swp_supported(page)	(!test_bit(PG_mte_tagged, &page->flags))

Are THP pages loaded from swap as a whole or are they split? IIRC the
splitting still happens but after the swapping out finishes. Even if
they are loaded as 4K pages, we still have the mte_save_tags() that only
understands small pages currently, so rejecting THP pages is probably
best.

-- 
Catalin