Messages in this thread | | | Date | Tue, 12 Dec 2023 17:35:23 +0100 | Subject | Re: [PATCH v9 04/10] mm: thp: Support allocation of anonymous multi-size THP | From | David Hildenbrand <> |
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On 12.12.23 16:38, Ryan Roberts wrote: > On 12/12/2023 15:02, David Hildenbrand wrote: >> On 07.12.23 17:12, Ryan Roberts 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. >> >> I'll note that with always-pte-mapped-thp it will be much easier to support >> incremental page clearing (e.g., zero only parts of the folio and map the >> remainder in a pro-non-like fashion whereby we'll zero on the next page fault). >> With a PMD-sized thp, you have to eventually place/rip out page tables to >> achieve that. > > But then you lose the benefits of reduced number of page faults; reducing page > faults gives a big speed up for workloads with lots of short lived processes > like compiling.
Well, you can do interesting things like "allocate order-5", but zero in order-3 chunks. You get less page faults and pay for alloc/rmap only once.
But yes, all has pros and cons.
[...]
>> >>> >>> 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. >>> >>> Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com> >>> Tested-by: John Hubbard <jhubbard@nvidia.com> >>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>> --- >>> include/linux/huge_mm.h | 6 ++- >>> mm/memory.c | 111 ++++++++++++++++++++++++++++++++++++---- >>> 2 files changed, 106 insertions(+), 11 deletions(-) >>> >>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >>> index 609c153bae57..fa7a38a30fc6 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) >> >> [...] >> >>> + >>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE >>> +static struct folio *alloc_anon_folio(struct vm_fault *vmf) >>> +{ >>> + struct vm_area_struct *vma = vmf->vma; >>> + unsigned long orders; >>> + struct folio *folio; >>> + unsigned long addr; >>> + pte_t *pte; >>> + gfp_t gfp; >>> + int order; >>> + >>> + /* >>> + * If uffd is active for the vma we need per-page fault fidelity to >>> + * maintain the uffd semantics. >>> + */ >>> + if (unlikely(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); >>> + >>> + /* >>> + * Find the highest order where the aligned range is completely >>> + * pte_none(). Note that all remaining orders will be completely >>> + * pte_none(). >>> + */ >>> + order = highest_order(orders); >>> + while (orders) { >>> + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order); >>> + if (pte_range_none(pte + pte_index(addr), 1 << order)) >>> + break; >>> + order = next_order(&orders, order); >>> + } >>> + >>> + pte_unmap(pte); >>> + >>> + /* Try allocating the highest of the remaining orders. */ >>> + 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, vmf->address, 1 << order); >>> + 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 >> >> A neater alternative might be >> >> static struct folio *alloc_anon_folio(struct vm_fault *vmf) >> { >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >> /* magic */ >> fallback: >> #endif >> return vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address): >> } > > I guess beauty lies in the eye of the beholder... I don't find it much neater > personally :). But happy to make the change if you insist; what's the process > now that its in mm-unstable? Just send a patch to Andrew for squashing?
That way it is clear that the fallback for thp is just what !thp does.
But either is fine for me; no need to change if you disagree.
-- Cheers,
David / dhildenb
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