Re: [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting

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

On Wed, Nov 8, 2023 at 2:05 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Tue, Nov 7, 2023 at 8:46 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >
> > On 04/11/2023 09:34, Barry Song wrote:
> > >> Yes that's right. mte_save_tags() needs to allocate memory so can fail
> > >> and if failing then arch_prepare_to_swap() would need to put things back
> > >> how they were with calls to mte_invalidate_tags() (although I think
> > >> you'd actually want to refactor to create a function which takes a
> > >> struct page *).
> > >>
> > >> Steve
> > >
> > > Thanks, Steve. combining all comments from You and Ryan, I made a v2.
> > > One tricky thing is that we are restoring one page rather than folio
> > > in arch_restore_swap() as we are only swapping in one page at this
> > > stage.
> > >
> > > [RFC v2 PATCH] arm64: mm: swap: save and restore mte tags for large folios
> > >
> > > This patch makes MTE tags saving and restoring support large folios,
> > > then we don't need to split them into base pages for swapping on
> > > ARM64 SoCs with MTE.
> > >
> > > This patch moves arch_prepare_to_swap() to take folio rather than
> > > page, as we support THP swap-out as a whole. And this patch also
> > > drops arch_thp_swp_supported() as ARM64 MTE is the only one who
> > > needs it.
> > >
> > > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > > ---
> > >  arch/arm64/include/asm/pgtable.h | 21 +++------------
> > >  arch/arm64/mm/mteswap.c          | 44 ++++++++++++++++++++++++++++++++
> > >  include/linux/huge_mm.h          | 12 ---------
> > >  include/linux/pgtable.h          |  2 +-
> > >  mm/page_io.c                     |  2 +-
> > >  mm/swap_slots.c                  |  2 +-
> > >  6 files changed, 51 insertions(+), 32 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > > index b19a8aee684c..d8f523dc41e7 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
> > > @@ -1036,12 +1030,8 @@ 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;
> > > -}
> > > +#define arch_prepare_to_swap arch_prepare_to_swap
> > > +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)
> > > @@ -1057,11 +1047,8 @@ 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);
> > > -}
> > > +#define arch_swap_restore arch_swap_restore
> > > +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..14a479e4ea8e 100644
> > > --- a/arch/arm64/mm/mteswap.c
> > > +++ b/arch/arm64/mm/mteswap.c
> > > @@ -68,6 +68,12 @@ 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 +89,41 @@ void mte_invalidate_tags_area(int type)
> > >       }
> > >       xa_unlock(&mte_pages);
> > >  }
> > > +
> > > +int arch_prepare_to_swap(struct folio *folio)
> > > +{
> > > +     int err;
> > > +     long i;
> > > +
> > > +     if (system_supports_mte()) {
> > > +             long nr = folio_nr_pages(folio);
> >
> > nit: there should be a clear line between variable declarations and logic.
>
> right.
>
> >
> > > +             for (i = 0; i < nr; i++) {
> > > +                     err = mte_save_tags(folio_page(folio, i));
> > > +                     if (err)
> > > +                             goto out;
> > > +             }
> > > +     }
> > > +     return 0;
> > > +
> > > +out:
> > > +     while (--i)
> >
> > If i is initially > 0, this will fail to invalidate page 0. If i is initially 0
> > then it will wrap and run ~forever. I think you meant `while (i--)`?
>
> nop. if i=0 and we goto out, that means the page0 has failed to save tags,
> there is nothing to revert. if i=3 and we goto out, that means 0,1,2 have
> saved, we restore 0,1,2 and we don't restore 3.

I am terribly sorry for my previous noise. You are right, Ryan. i
actually meant i--.

>
> >
> > > +             __mte_invalidate_tags(folio_page(folio, i));
> > > +     return err;
> > > +}
> > > +
> > > +void arch_swap_restore(swp_entry_t entry, struct folio *folio)
> > > +{
> > > +     if (system_supports_mte()) {
> > > +             /*
> > > +              * We don't support large folios swap in as whole yet, but
> > > +              * we can hit a large folio which is still in swapcache
> > > +              * after those related processes' PTEs have been unmapped
> > > +              * but before the swapcache folio  is dropped, in this case,
> > > +              * we need to find the exact page which "entry" is mapping
> > > +              * to. If we are not hitting swapcache, this folio won't be
> > > +              * large
> > > +              */
> >
> > So the currently defined API allows a large folio to be passed but the caller is
> > supposed to find the single correct page using the swap entry? That feels quite
> > nasty to me. And that's not what the old version of the function was doing; it
> > always assumed that the folio was small and passed the first page (which also
> > doesn't feel 'nice'). If the old version was wrong, I suggest a separate commit
> > to fix that. If the old version is correct, then I guess this version is wrong.
>
> the original version(mainline) is wrong but it works as once we find the SoCs
> support MTE, we will split large folios into small pages. so only small pages
> will be added into swapcache successfully.
>
> but now we want to swap out large folios even on SoCs with MTE as a whole,
> we don't split, so this breaks the assumption do_swap_page() will always get
> small pages.

let me clarify this more. The current mainline assumes
arch_swap_restore() always
get a folio with only one page. this is true as we split large folios
if we find SoCs
have MTE. but since we are dropping the split now, that means a large
folio can be
gotten by do_swap_page(). we have a chance that try_to_unmap_one() has been done
but folio is not put. so PTEs will have swap entry but folio is still
there, and do_swap_page()
to hit cache directly and the folio won't be released.

but after getting the large folio in do_swap_page, it still only takes
one basepage particularly
for the faulted PTE and maps this 4KB PTE only. so it uses the faulted
swap_entry and
the folio as parameters to call arch_swap_restore() which can be something like:

do_swap_page()
{
        arch_swap_restore(the swap entry for the faulted 4KB PTE, large folio);
}
>
> >
> > Thanks,
> > Ryan

Thanks
Barry