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

Ryan Roberts
2023-12-06T10:16:34Z

On 05/12/2023 20:16, Barry Song wrote:
> On Tue, Dec 5, 2023 at 11:48 PM Ryan Roberts <ryan.roberts@arm.com> 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.
>>
>> I have a vague recollection __GFP_ZERO is not preferred for large folios because
>> of some issue with virtually indexed caches? (Matthew: did I see you mention
>> that in some other context?)
>>
>> That said, I wasn't aware that Android ships with
>> CONFIG_INIT_ON_ALLOC_DEFAULT_ON (I thought it was only used as a debug option),
>> so I can see the potential for some overhead reduction here.
>>
>> Options:
>>
>> 1) leave it as is and accept the duplicated clearing
>> 2) Pass __GFP_ZERO and remove clear_huge_page()
>> 3) define __GFP_SKIP_ZERO even when kasan is not enabled and pass it down so
>> clear_huge_page() is the only clear
>> 4) make clear_huge_page() conditional on !want_init_on_alloc()
>>
>> I prefer option 4. What do you think?
>
> either 1 and 4 is ok to me if we will finally remove this duplicated
> clear_huge_page on top.
> 4 is even better as it can at least temporarily resolve the problem.

I'm going to stick with option 1 for this series. Then we can fix it uniformly
here and for PMD-sized THP in a separate patch (possibly with the approach
suggested in 4).

>
> in Android gki_defconfig,
> https://android.googlesource.com/kernel/common/+/refs/heads/android14-6.1-lts/arch/arm64/configs/gki_defconfig
>
> Android always has the below,
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
>
> here is some explanation for the reason,
> https://source.android.com/docs/security/test/memory-safety/zero-initialized-memory
>
>>
>> As an aside, I've also noticed that clear_huge_page() should take vmf->address
>> so that it clears the faulting page last to keep the cache hot. If we decide on
>> an option that keeps clear_huge_page(), I'll also make that change.

I'll make this change for the next version.

>>
>> Thanks,
>> Ryan
>>
>>>>
>
> Thanks
> Barry