Re: [PATCH] arm64: enable THP_SWAP for arm64

[Yang Shi <shy828301@gmail.com>]

On Thu, May 26, 2022 at 2:19 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Thu, May 26, 2022 at 5:49 AM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Wed, May 25, 2022 at 4:10 AM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Wed, May 25, 2022 at 7:14 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > >
> > > > 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
> > >
> > > i can confirm thp is written as a whole through:
> > > [   90.622863]  __swap_writepage+0xe8/0x580
> > > [   90.622881]  swap_writepage+0x44/0xf8
> > > [   90.622891]  pageout+0xe0/0x2a8
> > > [   90.622906]  shrink_page_list+0x9dc/0xde0
> > > [   90.622917]  shrink_inactive_list+0x1ec/0x3c8
> > > [   90.622928]  shrink_lruvec+0x3dc/0x628
> > > [   90.622939]  shrink_node+0x37c/0x6a0
> > > [   90.622950]  balance_pgdat+0x354/0x668
> > > [   90.622961]  kswapd+0x1e0/0x3c0
> > > [   90.622972]  kthread+0x110/0x120
> > >
> > > but i have never got a backtrace in which thp is loaded as a whole though it
> > > seems the code has this path:
> >
> > THP could be swapped out in a whole, but never swapped in as THP. Just
> > the single base page (4K on x86) is swapped in.
>
> yep. it seems swapin_readahead() is never reading a THP or even splitted
> pages for this 2MB THP.
>
> the number of pages to be read-ahead is determined either by
> /proc/sys/vm/page-cluster if /sys/kernel/mm/swap/vma_ra_enabled is fase
> or
> by vma read-ahead algorithm if /sys//kernel/mm/swap/vma_ra_enabled is true
> And the number is usually quite small.
>
> Am I missing any case in which 2MB can be swapped in as whole either by
> splitted pages or a THP?

Even though readahead swaps in 2MB, they are 512 single base pages
rather than THP. They may not be physically continuous at all.

>
> Thanks
> Barry