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

David Hildenbrand
2023-12-05T11:16:58Z

On 05.12.23 11:48, Ryan Roberts wrote:
> On 05/12/2023 01:24, Barry Song wrote:
>> On Tue, Dec 5, 2023 at 9:15 AM Barry Song <21cnbao@gmail.com> wrote:
>>>
>>> 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?
>
> I'm currently following the same allocation pattern as is done for PMD-sized
> THP. In earlier versions of this patch I was trying to be smarter and use the
> __GFP_ZERO/__GFP_ZEROTAGS as you suggest, but I was advised to keep it simple
> and follow the existing pattern.

Yes, this should be optimized on top IMHO.

--
Cheers,

David / dhildenb