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

[Ryan Roberts]

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 :)

>>>
>>> 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.

> 
>> 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.

> 
>>
>>> +             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