Re: [RFC PATCH v3 1/5] arm64: mm: swap: support THP_SWAP on hardware with MTE
From: Barry Song
Date: Fri Mar 22 2024 - 03:42:20 EST
On Fri, Mar 22, 2024 at 3:51 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2024 at 11:31 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 :)
> >
> > >>>
> > >>> 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.
>
> Hi Ryan,
>
> While working on the new version of this patch, I've come to recognize
> that, for the time being, it's
> imperative to maintain a natural alignment. The following code
> operates on the basis of this
> assumption.
>
> /**
> * folio_file_page - The page for a particular index.
> * @folio: The folio which contains this index.
> * @index: The index we want to look up.
> *
> * Sometimes after looking up a folio in the page cache, we need to
> * obtain the specific page for an index (eg a page fault).
> *
> * Return: The page containing the file data for this index.
> */
> static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
> {
> return folio_page(folio, index & (folio_nr_pages(folio) - 1));
> }
>
>
> It's invoked everywhere, particularly within do_swap_page(). Nonetheless,
> I remain confident that I can consistently pass the first entry to
> arch_swap_restore().
After grappling for a couple of hours, I've realized that the only
viable approach
is as follows: shifting the task of obtaining the first entry from the
callee to the
callers( looks silly). This is necessary due to various scenarios like
swap cache,
non-swap cache, and KSM, each presenting different cases. Since there's no
assurance of folio->swap being present, forcibly setting folio->swap could pose
risks (There might not even be any risk involved, but the associated
task getting
the first entry still cannot be overlooked by callers).
diff --git a/mm/internal.h b/mm/internal.h
index 7e486f2c502c..94d5b4b5a5da 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -76,6 +76,20 @@ static inline int folio_nr_pages_mapped(struct folio *folio)
return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
}
+/*
+ * Retrieve the first entry of a folio based on a provided entry within the
+ * folio. We cannot rely on folio->swap as there is no guarantee that it has
+ * been initialized. Used by arch_swap_restore()
+ */
+static inline swp_entry_t folio_swap(swp_entry_t entry, struct folio *folio)
+{
+ swp_entry_t swap = {
+ .val = entry.val & (folio_nr_pages(folio) - 1),
+ };
+
+ return swap;
+}
+
static inline void *folio_raw_mapping(struct folio *folio)
{
unsigned long mapping = (unsigned long)folio->mapping;
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..b7cab8be8632 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4188,7 +4188,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
* when reading from swap. This metadata may be indexed by swap entry
* so this must be called before swap_free().
*/
- arch_swap_restore(entry, folio);
+ arch_swap_restore(folio_swap(entry, folio), folio);
/*
* Remove the swap entry and conditionally try to free up the swapcache.
diff --git a/mm/shmem.c b/mm/shmem.c
index 0aad0d9a621b..82c9df4628f2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1913,7 +1913,7 @@ static int shmem_swapin_folio(struct inode
*inode, pgoff_t index,
* Some architectures may have to restore extra metadata to the
* folio after reading from swap.
*/
- arch_swap_restore(swap, folio);
+ arch_swap_restore(folio_swap(entry, folio), folio);
if (shmem_should_replace_folio(folio, gfp)) {
error = shmem_replace_folio(&folio, gfp, info, index);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4919423cce76..5e6d2304a2a4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1806,7 +1806,7 @@ static int unuse_pte(struct vm_area_struct *vma,
pmd_t *pmd,
* when reading from swap. This metadata may be indexed by swap entry
* so this must be called before swap_free().
*/
- arch_swap_restore(entry, folio);
+ arch_swap_restore(folio_swap(entry, folio), folio);
dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
Meanwhile, natural alignment is essential even during the execution of
add_to_swap(), as failure to
do so will trigger the VM_BUG_ON condition below.
int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
gfp_t gfp, void **shadowp)
{
struct address_space *address_space = swap_address_space(entry);
pgoff_t idx = swp_offset(entry);
XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));
unsigned long i, nr = folio_nr_pages(folio);
...
folio_set_swapcache(folio);
folio->swap = entry;
do {
xas_lock_irq(&xas);
xas_create_range(&xas);
if (xas_error(&xas))
goto unlock;
for (i = 0; i < nr; i++) {
VM_BUG_ON_FOLIO(xas.xa_index != idx + i, folio);
if (shadowp) {
old = xas_load(&xas);
if (xa_is_value(old))
*shadowp = old;
}
xas_store(&xas, folio);
xas_next(&xas);
}
}
Based on the information provided, Ryan, would it be feasible to retain the task
of obtaining the first entry within the callee? Or, are you in favor
of utilizing the
new folio_swap() helper?
>
> >
> > >
> > >> 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
> >