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

From: Barry Song
Date: Thu Mar 21 2024 - 06:44:18 EST

On Thu, Mar 21, 2024 at 6:31 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 21/03/2024 08:42, Barry Song wrote:
> > Hi Ryan,
> > Sorry for the late reply.
>
> No problem!
>
> >
> > 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.
>
> I think we should decouple this patch from your swap-in series. I suspect this
> one could be ready and go in sooner than the swap-in series based on the current
> discussions :)

I concur, particularly given that nowadays, most modern and popular ARM64
SoCs are equipped with MTE. The absence of this patch also hinders the
effective functioning of mTHP swap-out.

>
> >>>
> >>> 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 ?
>
> Yes, I agree about contiguity. My objection is about assuming natural alignment
> though. It can still be contiguous while not naturally aligned in swap.

right.

>
> >
> >> 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?
>
> Yes, if we must pass a folio here, I'd prefer that entry always corresponds to
> the first entry for the folio. That will remove the need for this function to do
> the alignment above too. So win-win.

right.

>
> >
> >>
> >>> + 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