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

From: Ryan Roberts
Date: Mon Aug 07 2023 - 15:08:09 EST

On 07/08/2023 06:24, Yu Zhao wrote:
> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx>
>> ---
>> 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?

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


Thanks,
Ryan