Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP

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

On Mon, Dec 4, 2023 at 6:21 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> Introduce the logic to allow THP to be configured (through the new sysfs
> interface we just added) to allocate large folios to back anonymous
> memory, which are larger than the base page size but smaller than
> PMD-size. We call this new THP extension "multi-size THP" (mTHP).
>
> mTHP continues to be PTE-mapped, but in many cases can still provide
> similar benefits to traditional PMD-sized THP: Page faults are
> significantly reduced (by a factor of e.g. 4, 8, 16, etc. depending on
> the configured order), but latency spikes are much less prominent
> because the size of each page isn't as huge as the PMD-sized variant and
> there is less memory to clear in each page fault. 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.
>
> Some architectures also employ TLB compression mechanisms to squeeze
> more entries in when a set of PTEs are virtually and physically
> contiguous and approporiately aligned. In this case, TLB misses will
> occur less often.
>
> The new behaviour is disabled by default, but can be enabled at runtime
> by writing to /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled
> (see documentation in previous commit). The long term aim is to change
> the default to include suitable lower orders, but there are some risks
> around internal fragmentation that need to be better understood first.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  include/linux/huge_mm.h |   6 ++-
>  mm/memory.c             | 106 ++++++++++++++++++++++++++++++++++++----
>  2 files changed, 101 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index bd0eadd3befb..91a53b9835a4 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -68,9 +68,11 @@ extern struct kobj_attribute shmem_enabled_attr;
>  #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>
>  /*
> - * Mask of all large folio orders supported for anonymous THP.
> + * Mask of all large folio orders supported for anonymous THP; all orders up to
> + * and including PMD_ORDER, except order-0 (which is not "huge") and order-1
> + * (which is a limitation of the THP implementation).
>   */
> -#define THP_ORDERS_ALL_ANON    BIT(PMD_ORDER)
> +#define THP_ORDERS_ALL_ANON    ((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
>
>  /*
>   * Mask of all large folio orders supported for file THP.
> diff --git a/mm/memory.c b/mm/memory.c
> index 3ceeb0f45bf5..bf7e93813018 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4125,6 +4125,84 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         return ret;
>  }
>
> +static bool pte_range_none(pte_t *pte, int nr_pages)
> +{
> +       int i;
> +
> +       for (i = 0; i < nr_pages; i++) {
> +               if (!pte_none(ptep_get_lockless(pte + i)))
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> +{
> +       gfp_t gfp;
> +       pte_t *pte;
> +       unsigned long addr;
> +       struct folio *folio;
> +       struct vm_area_struct *vma = vmf->vma;
> +       unsigned long orders;
> +       int order;
> +
> +       /*
> +        * If uffd is active for the vma we need per-page fault fidelity to
> +        * maintain the uffd semantics.
> +        */
> +       if (userfaultfd_armed(vma))
> +               goto fallback;
> +
> +       /*
> +        * Get a list of all the (large) orders below PMD_ORDER that are enabled
> +        * for this vma. Then filter out the orders that can't be allocated over
> +        * the faulting address and still be fully contained in the vma.
> +        */
> +       orders = thp_vma_allowable_orders(vma, vma->vm_flags, false, true, true,
> +                                         BIT(PMD_ORDER) - 1);
> +       orders = thp_vma_suitable_orders(vma, vmf->address, orders);
> +
> +       if (!orders)
> +               goto fallback;
> +
> +       pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> +       if (!pte)
> +               return ERR_PTR(-EAGAIN);
> +
> +       order = first_order(orders);
> +       while (orders) {
> +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> +               vmf->pte = pte + pte_index(addr);
> +               if (pte_range_none(vmf->pte, 1 << order))
> +                       break;
> +               order = next_order(&orders, order);
> +       }
> +
> +       vmf->pte = NULL;
> +       pte_unmap(pte);
> +
> +       gfp = vma_thp_gfp_mask(vma);
> +
> +       while (orders) {
> +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> +               folio = vma_alloc_folio(gfp, order, vma, addr, true);
> +               if (folio) {
> +                       clear_huge_page(&folio->page, addr, 1 << order);

Minor.

Do we have to constantly clear a huge page? Is it possible to let
post_alloc_hook()
finish this job by using __GFP_ZERO/__GFP_ZEROTAGS as
vma_alloc_zeroed_movable_folio() is doing?

struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
                                                unsigned long vaddr)
{
        gfp_t flags = GFP_HIGHUSER_MOVABLE | __GFP_ZERO;

        /*
         * If the page is mapped with PROT_MTE, initialise the tags at the
         * point of allocation and page zeroing as this is usually faster than
         * separate DC ZVA and STGM.
         */
        if (vma->vm_flags & VM_MTE)
                flags |= __GFP_ZEROTAGS;

        return vma_alloc_folio(flags, 0, vma, vaddr, false);
}

> +                       return folio;
> +               }
> +               order = next_order(&orders, order);
> +       }
> +
> +fallback:
> +       return vma_alloc_zeroed_movable_folio(vma, vmf->address);
> +}
> +#else
> +#define alloc_anon_folio(vmf) \
> +               vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address)
> +#endif
> +
>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -4132,6 +4210,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>   */
>  static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  {
> +       int i;
> +       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;
> @@ -4176,10 +4257,15 @@ 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);
> +       folio = alloc_anon_folio(vmf);
> +       if (IS_ERR(folio))
> +               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);
> @@ -4196,12 +4282,13 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>         if (vma->vm_flags & VM_WRITE)
>                 entry = pte_mkwrite(pte_mkdirty(entry), vma);
>
> -       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 ((nr_pages == 1 && vmf_pte_changed(vmf)) ||
> +           (nr_pages  > 1 && !pte_range_none(vmf->pte, nr_pages))) {
> +               for (i = 0; i < nr_pages; i++)
> +                       update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>                 goto release;
>         }
>
> @@ -4216,16 +4303,17 @@ 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);
>  setpte:
>         if (uffd_wp)
>                 entry = pte_mkuffd_wp(entry);
> -       set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> +       set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
>
>         /* No need to invalidate - it was non-present before */
> -       update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> +       update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
>  unlock:
>         if (vmf->pte)
>                 pte_unmap_unlock(vmf->pte, vmf->ptl);
> --
> 2.25.1
>