Re: [PATCH v3 3/4] mm: FLEXIBLE_THP for improved performance

[Yu Zhao <yuzhao@google.com>]

On Fri, Jul 14, 2023 at 10:17 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> Introduce FLEXIBLE_THP 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 FLEXIBLE_THP 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 by the new cmdline parameter,
> `flexthp_unhinted_max`. This allows for a performance boost without
> requiring any explicit opt-in from the workload while allowing the
> sysadmin to tune between performance and internal fragmentation.
>
> 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.
>
> 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.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  10 +
>  mm/Kconfig                                    |  10 +
>  mm/memory.c                                   | 187 ++++++++++++++++--
>  3 files changed, 190 insertions(+), 17 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a1457995fd41..405d624e2191 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1497,6 +1497,16 @@
>                         See Documentation/admin-guide/sysctl/net.rst for
>                         fb_tunnels_only_for_init_ns
>
> +       flexthp_unhinted_max=
> +                       [KNL] Requires CONFIG_FLEXIBLE_THP enabled. The maximum
> +                       folio size that will be allocated for an anonymous vma
> +                       that has neither explicitly opted in nor out of using
> +                       transparent hugepages. The size must be a power-of-2 in
> +                       the range [PAGE_SIZE, PMD_SIZE). A larger size improves
> +                       performance by reducing page faults, while a smaller
> +                       size reduces internal fragmentation. Default: max(64K,
> +                       PAGE_SIZE). Format: size[KMG].
> +

Let's split this parameter into a separate patch.

And I'm going to ask many questions about it (I can live with a sysctl
parameter but this boot parameter is unacceptable to me).

> diff --git a/mm/memory.c b/mm/memory.c
> index 01f39e8144ef..e8bc729efb9d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4050,6 +4050,148 @@ 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_FLEXIBLE_THP
> +static int flexthp_unhinted_max_order =
> +               ilog2(SZ_64K > PAGE_SIZE ? SZ_64K : PAGE_SIZE) - PAGE_SHIFT;
> +
> +static int __init parse_flexthp_unhinted_max(char *s)
> +{
> +       unsigned long long size = memparse(s, NULL);
> +
> +       if (!is_power_of_2(size) || size < PAGE_SIZE || size > PMD_SIZE) {
> +               pr_warn("flexthp: flexthp_unhinted_max=%s must be power-of-2 between PAGE_SIZE (%lu) and PMD_SIZE (%lu), ignoring\n",
> +                       s, PAGE_SIZE, PMD_SIZE);
> +               return 1;
> +       }
> +
> +       flexthp_unhinted_max_order = ilog2(size) - PAGE_SHIFT;
> +
> +       /* THP machinery requires at least 3 struct pages for meta data. */
> +       if (flexthp_unhinted_max_order == 1)
> +               flexthp_unhinted_max_order--;
> +
> +       return 1;
> +}
> +
> +__setup("flexthp_unhinted_max=", parse_flexthp_unhinted_max);
> +
> +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 the flexthp_unhinted_max cmdline
> +        * parameter. This allows a sysadmin to tune performance vs internal
> +        * fragmentation.
> +        */
> +
> +       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, flexthp_unhinted_max_order);
> +       }
> +
> +       return order;
> +}
> +
> +static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
> +{
> +       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;

It would be a bug if this happens. So probably -EINVAL?

> +
> +       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)

Drop "inline" (it doesn't do anything in .c).

The rest looks good to me.