Re: [PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance

[Yu Zhao]

On Wed, Jul 26, 2023 at 10:41 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >
> > Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be
> > allocated in large folios of a determined order. All pages of the large
> > folio are pte-mapped during the same page fault, significantly reducing
> > the number of page faults. The number of per-page operations (e.g. ref
> > counting, rmap management lru list management) are also significantly
> > reduced since those ops now become per-folio.
> >
> > The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig,
> > which defaults to disabled for now; The long term aim is for this to
> > defaut to enabled, but there are some risks around internal
> > fragmentation that need to be better understood first.
> >
> > When enabled, the folio order is determined as such: For a vma, process
> > or system that has explicitly disabled THP, we continue to allocate
> > order-0. THP is most likely disabled to avoid any possible internal
> > fragmentation so we honour that request.
> >
> > Otherwise, the return value of arch_wants_pte_order() is used. For vmas
> > that have not explicitly opted-in to use transparent hugepages (e.g.
> > where thp=madvise and the vma does not have MADV_HUGEPAGE), then
> > arch_wants_pte_order() is limited to 64K (or PAGE_SIZE, whichever is
> > bigger). This allows for a performance boost without requiring any
> > explicit opt-in from the workload while limitting internal
> > fragmentation.
> >
> > If the preferred order can't be used (e.g. because the folio would
> > breach the bounds of the vma, or because ptes in the region are already
> > mapped) then we fall back to a suitable lower order; first
> > PAGE_ALLOC_COSTLY_ORDER, then order-0.
> >
> > arch_wants_pte_order() can be overridden by the architecture if desired.
> > Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
> > set of ptes map physically contigious, naturally aligned memory, so this
> > mechanism allows the architecture to optimize as required.
> >
> > Here we add the default implementation of arch_wants_pte_order(), used
> > when the architecture does not define it, which returns -1, implying
> > that the HW has no preference. In this case, mm will choose it's own
> > default order.
> >
> > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > ---
> >  include/linux/pgtable.h |  13 ++++
> >  mm/Kconfig              |  10 +++
> >  mm/memory.c             | 166 ++++++++++++++++++++++++++++++++++++----
> >  3 files changed, 172 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 5063b482e34f..2a1d83775837 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -313,6 +313,19 @@ static inline bool arch_has_hw_pte_young(void)
> >  }
> >  #endif
> >
> > +#ifndef arch_wants_pte_order
> > +/*
> > + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
> > + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
> > + * to be at least order-2. Negative value implies that the HW has no preference
> > + * and mm will choose it's own default order.
> > + */
> > +static inline int arch_wants_pte_order(void)
> > +{
> > +       return -1;
> > +}
> > +#endif
> > +
> >  #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
> >  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >                                        unsigned long address,
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 09130434e30d..fa61ea160447 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -1238,4 +1238,14 @@ config LOCK_MM_AND_FIND_VMA
> >
> >  source "mm/damon/Kconfig"
> >
> > +config LARGE_ANON_FOLIO
> > +       bool "Allocate large folios for anonymous memory"
> > +       depends on TRANSPARENT_HUGEPAGE
> > +       default n
> > +       help
> > +         Use large (bigger than order-0) folios to back anonymous memory where
> > +         possible, even for pte-mapped memory. This reduces the number of page
> > +         faults, as well as other per-page overheads to improve performance for
> > +         many workloads.
> > +
> >  endmenu
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 01f39e8144ef..64c3f242c49a 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4050,6 +4050,127 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >         return ret;
> >  }
> >
> > +static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
> > +{
> > +       int i;
> > +
> > +       if (nr_pages == 1)
> > +               return vmf_pte_changed(vmf);
> > +
> > +       for (i = 0; i < nr_pages; i++) {
> > +               if (!pte_none(ptep_get_lockless(vmf->pte + i)))
> > +                       return true;
> > +       }
> > +
> > +       return false;
> > +}
> > +
> > +#ifdef CONFIG_LARGE_ANON_FOLIO
> > +#define ANON_FOLIO_MAX_ORDER_UNHINTED \
> > +               (ilog2(max_t(unsigned long, SZ_64K, PAGE_SIZE)) - PAGE_SHIFT)
> > +
> > +static int anon_folio_order(struct vm_area_struct *vma)
> > +{
> > +       int order;
> > +
> > +       /*
> > +        * If THP is explicitly disabled for either the vma, the process or the
> > +        * system, then this is very likely intended to limit internal
> > +        * fragmentation; in this case, don't attempt to allocate a large
> > +        * anonymous folio.
> > +        *
> > +        * Else, if the vma is eligible for thp, allocate a large folio of the
> > +        * size preferred by the arch. Or if the arch requested a very small
> > +        * size or didn't request a size, then use PAGE_ALLOC_COSTLY_ORDER,
> > +        * which still meets the arch's requirements but means we still take
> > +        * advantage of SW optimizations (e.g. fewer page faults).
> > +        *
> > +        * Finally if thp is enabled but the vma isn't eligible, take the
> > +        * arch-preferred size and limit it to ANON_FOLIO_MAX_ORDER_UNHINTED.
> > +        * This ensures workloads that have not explicitly opted-in take benefit
> > +        * while capping the potential for internal fragmentation.
> > +        */
>
> What empirical evidence is SZ_64K based on?
> What workloads would benefit from it?
> How much would they benefit from it?
> Would they benefit more or less from different values?
> How much internal fragmentation would it cause?
> What cost function was used to arrive at the conclusion that its
> benefits outweigh its costs?
>
> > +       if ((vma->vm_flags & VM_NOHUGEPAGE) ||
> > +           test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags) ||
> > +           !hugepage_flags_enabled())
> > +               order = 0;
> > +       else {
> > +               order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
> > +
> > +               if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
> > +                       order = min(order, ANON_FOLIO_MAX_ORDER_UNHINTED);
> > +       }

I'm a bit surprised to see the above: why can we overload existing
ABIs? I don't think we can. Assuming we could, you would have to
update Documentation/admin-guide/mm/transhuge.rst in the same
patchset, and the man page for madvise() in a separate patch.

Most importantly, existing userspace programs that don't work well
with THPs won't be able to use (try) large folios either -- this is a
big no no.



> > +
> > +       return order;
> > +}
> > +
> > +static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
>
> static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>
> and use ERR_PTR() and its friends.
>
> > +{
> > +       int i;
> > +       gfp_t gfp;
> > +       pte_t *pte;
> > +       unsigned long addr;
> > +       struct vm_area_struct *vma = vmf->vma;
> > +       int prefer = anon_folio_order(vma);
> > +       int orders[] = {
> > +               prefer,
> > +               prefer > PAGE_ALLOC_COSTLY_ORDER ? PAGE_ALLOC_COSTLY_ORDER : 0,
> > +               0,
> > +       };
> > +
> > +       *folio = NULL;
> > +
> > +       if (vmf_orig_pte_uffd_wp(vmf))
> > +               goto fallback;
> > +
> > +       for (i = 0; orders[i]; i++) {
> > +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > +               if (addr >= vma->vm_start &&
> > +                   addr + (PAGE_SIZE << orders[i]) <= vma->vm_end)
> > +                       break;
> > +       }
> > +
> > +       if (!orders[i])
> > +               goto fallback;
> > +
> > +       pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> > +       if (!pte)
> > +               return -EAGAIN;
> > +
> > +       for (; orders[i]; i++) {
> > +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > +               vmf->pte = pte + pte_index(addr);
> > +               if (!vmf_pte_range_changed(vmf, 1 << orders[i]))
> > +                       break;
> > +       }
> > +
> > +       vmf->pte = NULL;
> > +       pte_unmap(pte);
> > +
> > +       gfp = vma_thp_gfp_mask(vma);
> > +
> > +       for (; orders[i]; i++) {
> > +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > +               *folio = vma_alloc_folio(gfp, orders[i], vma, addr, true);
> > +               if (*folio) {
> > +                       clear_huge_page(&(*folio)->page, addr, 1 << orders[i]);
> > +                       return 0;
> > +               }
> > +       }
> > +
> > +fallback:
> > +       *folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> > +       return *folio ? 0 : -ENOMEM;
> > +}
> > +#else
> > +static inline int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
> > +{
> > +       *folio = vma_alloc_zeroed_movable_folio(vmf->vma, vmf->address);
> > +       return *folio ? 0 : -ENOMEM;
> > +}
> > +#endif
> > +
> >  /*
> >   * We enter with non-exclusive mmap_lock (to exclude vma changes,
> >   * but allow concurrent faults), and pte mapped but not yet locked.
> > @@ -4057,6 +4178,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >   */
> >  static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >  {
> > +       int i = 0;
> > +       int nr_pages = 1;
> > +       unsigned long addr = vmf->address;
> >         bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
> >         struct vm_area_struct *vma = vmf->vma;
> >         struct folio *folio;
> > @@ -4101,10 +4225,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >         /* Allocate our own private page. */
> >         if (unlikely(anon_vma_prepare(vma)))
> >                 goto oom;
> > -       folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> > +       ret = alloc_anon_folio(vmf, &folio);
> > +       if (unlikely(ret == -EAGAIN))
> > +               return 0;
> >         if (!folio)
> >                 goto oom;
> >
> > +       nr_pages = folio_nr_pages(folio);
> > +       addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
> > +
> >         if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
> >                 goto oom_free_page;
> >         folio_throttle_swaprate(folio, GFP_KERNEL);
> > @@ -4116,17 +4245,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >          */
> >         __folio_mark_uptodate(folio);
> >
> > -       entry = mk_pte(&folio->page, vma->vm_page_prot);
> > -       entry = pte_sw_mkyoung(entry);
> > -       if (vma->vm_flags & VM_WRITE)
> > -               entry = pte_mkwrite(pte_mkdirty(entry));
> > -
> > -       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> > -                       &vmf->ptl);
> > +       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> >         if (!vmf->pte)
> >                 goto release;
> > -       if (vmf_pte_changed(vmf)) {
> > -               update_mmu_tlb(vma, vmf->address, vmf->pte);
> > +       if (vmf_pte_range_changed(vmf, nr_pages)) {
> > +               for (i = 0; i < nr_pages; i++)
> > +                       update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> >                 goto release;
> >         }
> >
> > @@ -4141,16 +4265,24 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >                 return handle_userfault(vmf, VM_UFFD_MISSING);
> >         }
> >
> > -       inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > -       folio_add_new_anon_rmap(folio, vma, vmf->address);
> > +       folio_ref_add(folio, nr_pages - 1);
> > +       add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> > +       folio_add_new_anon_rmap(folio, vma, addr);
> >         folio_add_lru_vma(folio, vma);
> > +
> > +       for (i = 0; i < nr_pages; i++) {
> > +               entry = mk_pte(folio_page(folio, i), vma->vm_page_prot);
> > +               entry = pte_sw_mkyoung(entry);
> > +               if (vma->vm_flags & VM_WRITE)
> > +                       entry = pte_mkwrite(pte_mkdirty(entry));
> >  setpte:
> > -       if (uffd_wp)
> > -               entry = pte_mkuffd_wp(entry);
> > -       set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> > +               if (uffd_wp)
> > +                       entry = pte_mkuffd_wp(entry);
> > +               set_pte_at(vma->vm_mm, addr + PAGE_SIZE * i, vmf->pte + i, entry);
> >
> > -       /* No need to invalidate - it was non-present before */
> > -       update_mmu_cache(vma, vmf->address, vmf->pte);
> > +               /* No need to invalidate - it was non-present before */
> > +               update_mmu_cache(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> > +       }
> >  unlock:
> >         if (vmf->pte)
> >                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>
> The rest looks good to me.