Re: [PATCH] arm64: enable THP_SWAP for arm64

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

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:
> > 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.
> >
> > Cc: "Huang, Ying" <ying.huang@intel.com>
> > Cc: Minchan Kim <minchan@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Shaohua Li <shli@kernel.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Andrea Arcangeli <aarcange@redhat.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  arch/arm64/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > 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?

> 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;
        }

> --
> Catalin

Thanks
Barry