Re: [PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance

[Ryan Roberts <ryan.roberts@arm.com>]

On 02/08/2023 22:05, Yu Zhao wrote:
> On Wed, Aug 2, 2023 at 3:33 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 01/08/2023 07:18, Yu Zhao wrote:
>>> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> Introduce LARGE_ANON_FOLIO 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 LARGE_ANON_FOLIO 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 to 64K (or PAGE_SIZE, whichever is
>>>> bigger). This allows for a performance boost without requiring any
>>>> explicit opt-in from the workload while limitting internal
>>>> fragmentation.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> Here we add the default implementation of arch_wants_pte_order(), used
>>>> when the architecture does not define it, which returns -1, implying
>>>> that the HW has no preference. In this case, mm will choose it's own
>>>> default order.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>> ---
>>>>  include/linux/pgtable.h |  13 ++++
>>>>  mm/Kconfig              |  10 +++
>>>>  mm/memory.c             | 166 ++++++++++++++++++++++++++++++++++++----
>>>>  3 files changed, 172 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>> index 5063b482e34f..2a1d83775837 100644
>>>> --- a/include/linux/pgtable.h
>>>> +++ b/include/linux/pgtable.h
>>>> @@ -313,6 +313,19 @@ static inline bool arch_has_hw_pte_young(void)
>>>>  }
>>>>  #endif
>>>>
>>>> +#ifndef arch_wants_pte_order
>>>> +/*
>>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
>>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
>>>> + * to be at least order-2. Negative value implies that the HW has no preference
>>>> + * and mm will choose it's own default order.
>>>> + */
>>>> +static inline int arch_wants_pte_order(void)
>>>> +{
>>>> +       return -1;
>>>> +}
>>>> +#endif
>>>> +
>>>>  #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>>>>  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>>>>                                        unsigned long address,
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 09130434e30d..fa61ea160447 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -1238,4 +1238,14 @@ config LOCK_MM_AND_FIND_VMA
>>>>
>>>>  source "mm/damon/Kconfig"
>>>>
>>>> +config LARGE_ANON_FOLIO
>>>> +       bool "Allocate large folios for anonymous memory"
>>>> +       depends on TRANSPARENT_HUGEPAGE
>>>> +       default n
>>>> +       help
>>>> +         Use large (bigger than order-0) folios to back anonymous memory where
>>>> +         possible, even for pte-mapped memory. This reduces the number of page
>>>> +         faults, as well as other per-page overheads to improve performance for
>>>> +         many workloads.
>>>> +
>>>>  endmenu
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 01f39e8144ef..64c3f242c49a 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -4050,6 +4050,127 @@ 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_LARGE_ANON_FOLIO
>>>> +#define ANON_FOLIO_MAX_ORDER_UNHINTED \
>>>> +               (ilog2(max_t(unsigned long, SZ_64K, PAGE_SIZE)) - PAGE_SHIFT)
>>>> +
>>>> +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 ANON_FOLIO_MAX_ORDER_UNHINTED.
>>>> +        * This ensures workloads that have not explicitly opted-in take benefit
>>>> +        * while capping the potential for 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, ANON_FOLIO_MAX_ORDER_UNHINTED);
>>>> +       }
>>>> +
>>>> +       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;
>>>
>>> I think we need to s/vmf_orig_pte_uffd_wp/userfaultfd_armed/ here;
>>> otherwise UFFD would miss VM_UFFD_MISSING/MINOR.
>>
>> I don't think this is the case. As far as I can see, VM_UFFD_MINOR only applies
>> to shmem and hugetlb.
> 
> Correct, but we don't have a helper to check against (VM_UFFD_WP |
> VM_UFFD_MISSING). Reusing userfaultfd_armed() seems cleaner to me or
> even future proof.
> 
>> VM_UFFD_MISSING is checked under the PTL and if set on the
>> VMA, then it is handled without mapping the folio that was just allocated:
>>
>>         /* Deliver the page fault to userland, check inside PT lock */
>>         if (userfaultfd_missing(vma)) {
>>                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>>                 folio_put(folio);
>>                 return handle_userfault(vmf, VM_UFFD_MISSING);
>>         }
>>
>> So we are racing to allocate a large folio; if the vma later turns out to have
>> MISSING handling registered, we drop the folio and handle it, else we map the
>> large folio.
> 
> Yes, then we have allocated a large folio (with great effort if under
> heavy memory pressure) for nothing.
> 
>> The vmf_orig_pte_uffd_wp() *is* required because we need to individually check
>> each PTE for the uffd_wp bit and fix it up.
> 
> This is not correct: we cannot see a WP PTE before you see
> VM_UFFD_WP. So checking VM_UFFD_WP is perfectly safe.

I think you misunderstood me; I was trying to say that assuming we don't check
userfaultfd_armed() then we need the vmf_orig_pte_uffd_wp() check because we
need to ensure that the marker gets preserved for that specific pte and we can
only do that if we are operating on a single pte.

> 
> The reason we might want to check individual PTEs is because WP can be
> done to a subrange of a VMA that has VM_UFFD_WP, which I don't think
> is the common case and worth considering here. But if you want to keep
> it, that's fine with me. Without some comments, the next person might
> find these two checks confusing though, if you plan to add both.

I'm not proposing we need both checks.

> 
>> So I think the code is correct, but perhaps it is safer/simpler to always avoid
>> allocating a large folio if the vma is registered for uffd in the way you
>> suggest? I don't know enough about uffd to form a strong opinion either way.
> 
> Yes, it's not about correctness. Just a second thought about not
> allocating large folios unnecessarily when possible.

OK, I misunderstood you; I thought your original point is about correctness.

Anyway, you have convinced me that we should
s/vmf_orig_pte_uffd_wp/userfaultfd_armed/ on the grounds that trying hard to
allocate a high order folio is almost always going to be a waste of effort. I'll
change this in the next version.

Thanks,
Ryan