On Thu, May 26, 2022 at 2:19 AM Barry Song [off-list ref] wrote:

On Thu, May 26, 2022 at 5:49 AM Yang Shi [off-list ref] wrote:

quoted

On Wed, May 25, 2022 at 4:10 AM Barry Song [off-list ref] wrote:

quoted

On Wed, May 25, 2022 at 7:14 AM Catalin Marinas [off-list ref] wrote:

quoted

On Tue, May 24, 2022 at 10:05:35PM +1200, Barry Song wrote:

quoted

On Tue, May 24, 2022 at 8:12 PM Catalin Marinas [off-list ref] wrote:

quoted

On Tue, May 24, 2022 at 07:14:03PM +1200, Barry Song wrote:

quoted

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.

quoted

quoted

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

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