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

Barry Song
2023-12-05T01:16:30Z

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