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

Ryan Roberts
2023-12-05T10:49:15Z

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?

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.

Thanks,
Ryan

>>
>> 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);
>> }
>
> I am asking this because Android and some other kernels might always set
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON, that means one more explicit
> clear_page is doing a duplicated job.
>
> when the below is true, post_alloc_hook has cleared huge_page before
> vma_alloc_folio() returns the folio,
>
> static inline bool want_init_on_alloc(gfp_t flags)
> {
> if (static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
> &init_on_alloc))
> return true;
> return flags & __GFP_ZERO;
> }
>
>
>>
>>> + 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
>>>