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

[Yu Zhao <yuzhao@google.com>]

On Mon, Aug 7, 2023 at 1:07 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 07/08/2023 06:24, 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;
> >
> > Per the discussion, we need to check hugepage_vma_check() for
> > correctness of VM LM. I'd just check it here and fall back to order 0
> > if that helper returns false.
>
> I'm not sure if either you haven't noticed the logic in anon_folio_order()
> above, or whether you are making this suggestion because you disagree with the
> subtle difference in my logic?

The latter, or more generally the policy you described earlier.

> My logic is deliberately not calling hugepage_vma_check() because that would
> return false for the thp=madvise,mmap=unhinted case, whereas the policy I'm
> implementing wants to apply LAF in that case.
>
>
> My intended policy:
>
>                 | never     | madvise   | always
> ----------------|-----------|-----------|-----------
> no hint         | S         | LAF>S     | THP>LAF>S
> MADV_HUGEPAGE   | S         | THP>LAF>S | THP>LAF>S
> MADV_NOHUGEPAGE | S         | S         | S
>
>
> What your suggestion would give:
>
>                 | never     | madvise   | always
> ----------------|-----------|-----------|-----------
> no hint         | S         | S         | THP>LAF>S
> MADV_HUGEPAGE   | S         | THP>LAF>S | THP>LAF>S
> MADV_NOHUGEPAGE | S         | S         | S

This is not what I'm suggesting.

Let me reiterate [1]:
  My impression is we only agreed on one thing: at the current stage, we
  should respect things we absolutely have to. We didn't agree on what
  "never" means ("never 2MB" or "never >4KB"), and we didn't touch on
  how "always" should behave at all.

And [2]:
  (Thanks to David, now I agree that) we have to interpret MADV_NOHUGEPAGE
  as nothing >4KB.

My final take [3]:
  I agree these points require more discussion. But I don't think we
  need to conclude them now, unless they cause correctness issues like
  ignoring MADV_NOHUGEPAGE would.

But I should have been clear about the parameters to
hugepage_vma_check(): enforce_sysfs=false.

[1] https://lore.kernel.org/linux-mm/CAOUHufYQTcOdKU=1mPq-fdLV7a66sHx1=EJpPpMVogciCNKO9A@mail.gmail.com/
[2] https://lore.kernel.org/linux-mm/CAOUHufafd4GNna2GKdSyQdW6CLVh0gxhNgeOc6t+ZOphwgw7tw@mail.gmail.com/
[3] https://lore.kernel.org/linux-mm/CAOUHufYQTcOdKU=1mPq-fdLV7a66sHx1=EJpPpMVogciCNKO9A@mail.gmail.com/