Re: [RFC PATCH v3 1/5] arm64: mm: swap: support THP_SWAP on hardware with MTE

[Barry Song]

Hi Ryan,
Sorry for the late reply.

On Tue, Mar 12, 2024 at 5:56 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 04/03/2024 08:13, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > Commit d0637c505f8a1 ("arm64: enable THP_SWAP for arm64") brings up
> > THP_SWAP on ARM64, but it doesn't enable THP_SWP on hardware with
> > MTE as the MTE code works with the assumption tags save/restore is
> > always handling a folio with only one page.
> >
> > The limitation should be removed as more and more ARM64 SoCs have
> > this feature. Co-existence of MTE and THP_SWAP becomes more and
> > more important.
> >
> > This patch makes MTE tags saving support large folios, then we don't
> > need to split large folios into base pages for swapping out on ARM64
> > SoCs with MTE any more.
> >
> > arch_prepare_to_swap() should take folio rather than page as parameter
> > because we support THP swap-out as a whole. It saves tags for all
> > pages in a large folio.
> >
> > As now we are restoring tags based-on folio, in arch_swap_restore(),
> > we may increase some extra loops and early-exitings while refaulting
> > a large folio which is still in swapcache in do_swap_page(). In case
> > a large folio has nr pages, do_swap_page() will only set the PTE of
> > the particular page which is causing the page fault.
> > Thus do_swap_page() runs nr times, and each time, arch_swap_restore()
> > will loop nr times for those subpages in the folio. So right now the
> > algorithmic complexity becomes O(nr^2).
> >
> > Once we support mapping large folios in do_swap_page(), extra loops
> > and early-exitings will decrease while not being completely removed
> > as a large folio might get partially tagged in corner cases such as,
> > 1. a large folio in swapcache can be partially unmapped, thus, MTE
> > tags for the unmapped pages will be invalidated;
> > 2. users might use mprotect() to set MTEs on a part of a large folio.
> >
> > arch_thp_swp_supported() is dropped since ARM64 MTE was the only one
> > who needed it.
> >
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> > Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > Cc: Peter Collingbourne <pcc@xxxxxxxxxx>
> > Cc: Steven Price <steven.price@xxxxxxx>
> > Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > Cc: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > CC: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
> > Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > Reviewed-by: Steven Price <steven.price@xxxxxxx>
> > Acked-by: Chris Li <chrisl@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 19 ++------------
> >  arch/arm64/mm/mteswap.c          | 43 ++++++++++++++++++++++++++++++++
> >  include/linux/huge_mm.h          | 12 ---------
> >  include/linux/pgtable.h          |  2 +-
> >  mm/page_io.c                     |  2 +-
> >  mm/swap_slots.c                  |  2 +-
> >  6 files changed, 48 insertions(+), 32 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 401087e8a43d..7a54750770b8 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -45,12 +45,6 @@
> >       __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
> > @@ -1095,12 +1089,7 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> >  #ifdef CONFIG_ARM64_MTE
> >
> >  #define __HAVE_ARCH_PREPARE_TO_SWAP
> > -static inline int arch_prepare_to_swap(struct page *page)
> > -{
> > -     if (system_supports_mte())
> > -             return mte_save_tags(page);
> > -     return 0;
> > -}
> > +extern int arch_prepare_to_swap(struct folio *folio);
> >
> >  #define __HAVE_ARCH_SWAP_INVALIDATE
> >  static inline void arch_swap_invalidate_page(int type, pgoff_t offset)
> > @@ -1116,11 +1105,7 @@ static inline void arch_swap_invalidate_area(int type)
> >  }
> >
> >  #define __HAVE_ARCH_SWAP_RESTORE
> > -static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
> > -{
> > -     if (system_supports_mte())
> > -             mte_restore_tags(entry, &folio->page);
> > -}
> > +extern void arch_swap_restore(swp_entry_t entry, struct folio *folio);
> >
> >  #endif /* CONFIG_ARM64_MTE */
> >
> > diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
> > index a31833e3ddc5..295836fef620 100644
> > --- a/arch/arm64/mm/mteswap.c
> > +++ b/arch/arm64/mm/mteswap.c
> > @@ -68,6 +68,13 @@ void mte_invalidate_tags(int type, pgoff_t offset)
> >       mte_free_tag_storage(tags);
> >  }
> >
> > +static inline void __mte_invalidate_tags(struct page *page)
> > +{
> > +     swp_entry_t entry = page_swap_entry(page);
> > +
> > +     mte_invalidate_tags(swp_type(entry), swp_offset(entry));
> > +}
> > +
> >  void mte_invalidate_tags_area(int type)
> >  {
> >       swp_entry_t entry = swp_entry(type, 0);
> > @@ -83,3 +90,39 @@ void mte_invalidate_tags_area(int type)
> >       }
> >       xa_unlock(&mte_pages);
> >  }
> > +
> > +int arch_prepare_to_swap(struct folio *folio)
> > +{
> > +     long i, nr;
> > +     int err;
> > +
> > +     if (!system_supports_mte())
> > +             return 0;
> > +
> > +     nr = folio_nr_pages(folio);
> > +
> > +     for (i = 0; i < nr; i++) {
> > +             err = mte_save_tags(folio_page(folio, i));
> > +             if (err)
> > +                     goto out;
> > +     }
> > +     return 0;
> > +
> > +out:
> > +     while (i--)
> > +             __mte_invalidate_tags(folio_page(folio, i));
> > +     return err;
> > +}
> > +
> > +void arch_swap_restore(swp_entry_t entry, struct folio *folio)
>
> I'm still not a fan of the fact that entry could be anywhere within folio.
>
> > +{
> > +     if (system_supports_mte()) {
>
> nit: if you do:
>
> if (!system_supports_mte())
>         return;

Acked

>
> It will be consistent with arch_prepare_to_swap() and reduce the indentation of
> the main body.
>
> > +             long i, nr = folio_nr_pages(folio);
> > +
> > +             entry.val -= swp_offset(entry) & (nr - 1);
>
> This assumes that folios are always stored in swap with natural alignment Is
> that definitely a safe assumption? My swap-out series is currently ensuring that
> folios are swapped-out naturally aligned, but that is an implementation detail.
>

I concur that this is an implementation detail. However, we should be
bold enough
to state that swap slots will be contiguous, considering we are
currently utilizing
folio->swap instead of subpage->swap ?

> Your cover note for swap-in says that you could technically swap in a large
> folio without it having been swapped-out large. If you chose to do that in
> future, this would break, right? I don't think it's good to couple the swap

Right. technically I agree. Given that we still have many tasks involving even
swapping in contiguous swap slots, it's unlikely that swapping in large folios
for non-contiguous entries will occur in the foreseeable future :-)

> storage layout to the folio order that you want to swap into. Perhaps that's an
> argument for passing each *page* to this function with its exact, corresponding
> swap entry?

I recall Matthew Wilcox strongly objected to using "page" as the
parameter, so I've
discarded that approach. Alternatively, it appears I can consistently pass
folio->swap to this function and ensure the function always retrieves
the first entry?

>
> > +             for (i = 0; i < nr; i++) {
> > +                     mte_restore_tags(entry, folio_page(folio, i));
> > +                     entry.val++;
> > +             }
> > +     }
> > +}
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index de0c89105076..e04b93c43965 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -535,16 +535,4 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
> >  #define split_folio_to_list(f, l) split_folio_to_list_to_order(f, l, 0)
> >  #define split_folio(f) split_folio_to_order(f, 0)
> >
> > -/*
> > - * 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/include/linux/pgtable.h b/include/linux/pgtable.h
> > index e1b22903f709..bfcfe3386934 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1106,7 +1106,7 @@ static inline int arch_unmap_one(struct mm_struct *mm,
> >   * prototypes must be defined in the arch-specific asm/pgtable.h file.
> >   */
> >  #ifndef __HAVE_ARCH_PREPARE_TO_SWAP
> > -static inline int arch_prepare_to_swap(struct page *page)
> > +static inline int arch_prepare_to_swap(struct folio *folio)
> >  {
> >       return 0;
> >  }
> > diff --git a/mm/page_io.c b/mm/page_io.c
> > index ae2b49055e43..a9a7c236aecc 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -189,7 +189,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> >        * Arch code may have to preserve more data than just the page
> >        * contents, e.g. memory tags.
> >        */
> > -     ret = arch_prepare_to_swap(&folio->page);
> > +     ret = arch_prepare_to_swap(folio);
> >       if (ret) {
> >               folio_mark_dirty(folio);
> >               folio_unlock(folio);
> > diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> > index 90973ce7881d..53abeaf1371d 100644
> > --- a/mm/swap_slots.c
> > +++ b/mm/swap_slots.c
> > @@ -310,7 +310,7 @@ swp_entry_t folio_alloc_swap(struct folio *folio)
> >       entry.val = 0;
> >
> >       if (folio_test_large(folio)) {
> > -             if (IS_ENABLED(CONFIG_THP_SWAP) && arch_thp_swp_supported())
> > +             if (IS_ENABLED(CONFIG_THP_SWAP))
> >                       get_swap_pages(1, &entry, folio_nr_pages(folio));
> >               goto out;
> >       }
>

Thanks
Barry