Messages in this thread | | | Date | Thu, 16 Nov 2023 15:15:48 +0100 | Subject | Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork() | From | David Hildenbrand <> |
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On 16.11.23 15:13, David Hildenbrand wrote: > On 16.11.23 14:49, Ryan Roberts wrote: >> On 16/11/2023 13:20, David Hildenbrand wrote: >>> On 16.11.23 12:20, Ryan Roberts wrote: >>>> On 16/11/2023 11:03, David Hildenbrand wrote: >>>>> On 15.11.23 17:30, Ryan Roberts wrote: >>>>>> Convert copy_pte_range() to copy a set of ptes in a batch. A given batch >>>>>> maps a physically contiguous block of memory, all belonging to the same >>>>>> folio, with the same permissions, and for shared mappings, the same >>>>>> dirty state. This will likely improve performance by a tiny amount due >>>>>> to batching the folio reference count management and calling set_ptes() >>>>>> rather than making individual calls to set_pte_at(). >>>>>> >>>>>> However, the primary motivation for this change is to reduce the number >>>>>> of tlb maintenance operations that the arm64 backend has to perform >>>>>> during fork, as it is about to add transparent support for the >>>>>> "contiguous bit" in its ptes. By write-protecting the parent using the >>>>>> new ptep_set_wrprotects() (note the 's' at the end) function, the >>>>>> backend can avoid having to unfold contig ranges of PTEs, which is >>>>>> expensive, when all ptes in the range are being write-protected. >>>>>> Similarly, by using set_ptes() rather than set_pte_at() to set up ptes >>>>>> in the child, the backend does not need to fold a contiguous range once >>>>>> they are all populated - they can be initially populated as a contiguous >>>>>> range in the first place. >>>>>> >>>>>> This change addresses the core-mm refactoring only, and introduces >>>>>> ptep_set_wrprotects() with a default implementation that calls >>>>>> ptep_set_wrprotect() for each pte in the range. A separate change will >>>>>> implement ptep_set_wrprotects() in the arm64 backend to realize the >>>>>> performance improvement as part of the work to enable contpte mappings. >>>>>> >>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>>>>> --- >>>>>> include/linux/pgtable.h | 13 +++ >>>>>> mm/memory.c | 175 +++++++++++++++++++++++++++++++--------- >>>>>> 2 files changed, 150 insertions(+), 38 deletions(-) >>>>>> >>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>>>>> index af7639c3b0a3..1c50f8a0fdde 100644 >>>>>> --- a/include/linux/pgtable.h >>>>>> +++ b/include/linux/pgtable.h >>>>>> @@ -622,6 +622,19 @@ static inline void ptep_set_wrprotect(struct mm_struct >>>>>> *mm, unsigned long addres >>>>>> } >>>>>> #endif >>>>>> +#ifndef ptep_set_wrprotects >>>>>> +struct mm_struct; >>>>>> +static inline void ptep_set_wrprotects(struct mm_struct *mm, >>>>>> + unsigned long address, pte_t *ptep, >>>>>> + unsigned int nr) >>>>>> +{ >>>>>> + unsigned int i; >>>>>> + >>>>>> + for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++) >>>>>> + ptep_set_wrprotect(mm, address, ptep); >>>>>> +} >>>>>> +#endif >>>>>> + >>>>>> /* >>>>>> * On some architectures hardware does not set page access bit when >>>>>> accessing >>>>>> * memory page, it is responsibility of software setting this bit. It brings >>>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>>> index 1f18ed4a5497..b7c8228883cf 100644 >>>>>> --- a/mm/memory.c >>>>>> +++ b/mm/memory.c >>>>>> @@ -921,46 +921,129 @@ copy_present_page(struct vm_area_struct *dst_vma, >>>>>> struct vm_area_struct *src_vma >>>>>> /* Uffd-wp needs to be delivered to dest pte as well */ >>>>>> pte = pte_mkuffd_wp(pte); >>>>>> set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte); >>>>>> - return 0; >>>>>> + return 1; >>>>>> +} >>>>>> + >>>>>> +static inline unsigned long page_cont_mapped_vaddr(struct page *page, >>>>>> + struct page *anchor, unsigned long anchor_vaddr) >>>>>> +{ >>>>>> + unsigned long offset; >>>>>> + unsigned long vaddr; >>>>>> + >>>>>> + offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT; >>>>>> + vaddr = anchor_vaddr + offset; >>>>>> + >>>>>> + if (anchor > page) { >>>>>> + if (vaddr > anchor_vaddr) >>>>>> + return 0; >>>>>> + } else { >>>>>> + if (vaddr < anchor_vaddr) >>>>>> + return ULONG_MAX; >>>>>> + } >>>>>> + >>>>>> + return vaddr; >>>>>> +} >>>>>> + >>>>>> +static int folio_nr_pages_cont_mapped(struct folio *folio, >>>>>> + struct page *page, pte_t *pte, >>>>>> + unsigned long addr, unsigned long end, >>>>>> + pte_t ptent, bool *any_dirty) >>>>>> +{ >>>>>> + int floops; >>>>>> + int i; >>>>>> + unsigned long pfn; >>>>>> + pgprot_t prot; >>>>>> + struct page *folio_end; >>>>>> + >>>>>> + if (!folio_test_large(folio)) >>>>>> + return 1; >>>>>> + >>>>>> + folio_end = &folio->page + folio_nr_pages(folio); >>>>>> + end = min(page_cont_mapped_vaddr(folio_end, page, addr), end); >>>>>> + floops = (end - addr) >> PAGE_SHIFT; >>>>>> + pfn = page_to_pfn(page); >>>>>> + prot = pte_pgprot(pte_mkold(pte_mkclean(ptent))); >>>>>> + >>>>>> + *any_dirty = pte_dirty(ptent); >>>>>> + >>>>>> + pfn++; >>>>>> + pte++; >>>>>> + >>>>>> + for (i = 1; i < floops; i++) { >>>>>> + ptent = ptep_get(pte); >>>>>> + ptent = pte_mkold(pte_mkclean(ptent)); >>>>>> + >>>>>> + if (!pte_present(ptent) || pte_pfn(ptent) != pfn || >>>>>> + pgprot_val(pte_pgprot(ptent)) != pgprot_val(prot)) >>>>>> + break; >>>>>> + >>>>>> + if (pte_dirty(ptent)) >>>>>> + *any_dirty = true; >>>>>> + >>>>>> + pfn++; >>>>>> + pte++; >>>>>> + } >>>>>> + >>>>>> + return i; >>>>>> } >>>>>> /* >>>>>> - * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page >>>>>> - * is required to copy this pte. >>>>>> + * Copy set of contiguous ptes. Returns number of ptes copied if succeeded >>>>>> + * (always gte 1), or -EAGAIN if one preallocated page is required to copy the >>>>>> + * first pte. >>>>>> */ >>>>>> static inline int >>>>>> -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct >>>>>> *src_vma, >>>>>> - pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss, >>>>>> - struct folio **prealloc) >>>>>> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct >>>>>> *src_vma, >>>>>> + pte_t *dst_pte, pte_t *src_pte, >>>>>> + unsigned long addr, unsigned long end, >>>>>> + int *rss, struct folio **prealloc) >>>>>> { >>>>>> struct mm_struct *src_mm = src_vma->vm_mm; >>>>>> unsigned long vm_flags = src_vma->vm_flags; >>>>>> pte_t pte = ptep_get(src_pte); >>>>>> struct page *page; >>>>>> struct folio *folio; >>>>>> + int nr = 1; >>>>>> + bool anon; >>>>>> + bool any_dirty = pte_dirty(pte); >>>>>> + int i; >>>>>> page = vm_normal_page(src_vma, addr, pte); >>>>>> - if (page) >>>>>> + if (page) { >>>>>> folio = page_folio(page); >>>>>> - if (page && folio_test_anon(folio)) { >>>>>> - /* >>>>>> - * If this page may have been pinned by the parent process, >>>>>> - * copy the page immediately for the child so that we'll always >>>>>> - * guarantee the pinned page won't be randomly replaced in the >>>>>> - * future. >>>>>> - */ >>>>>> - folio_get(folio); >>>>>> - if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) { >>>>>> - /* Page may be pinned, we have to copy. */ >>>>>> - folio_put(folio); >>>>>> - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte, >>>>>> - addr, rss, prealloc, page); >>>>>> + anon = folio_test_anon(folio); >>>>>> + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, >>>>>> + end, pte, &any_dirty); >>>>>> + >>>>>> + for (i = 0; i < nr; i++, page++) { >>>>>> + if (anon) { >>>>>> + /* >>>>>> + * If this page may have been pinned by the >>>>>> + * parent process, copy the page immediately for >>>>>> + * the child so that we'll always guarantee the >>>>>> + * pinned page won't be randomly replaced in the >>>>>> + * future. >>>>>> + */ >>>>>> + if (unlikely(page_try_dup_anon_rmap( >>>>>> + page, false, src_vma))) { >>>>>> + if (i != 0) >>>>>> + break; >>>>>> + /* Page may be pinned, we have to copy. */ >>>>>> + return copy_present_page( >>>>>> + dst_vma, src_vma, dst_pte, >>>>>> + src_pte, addr, rss, prealloc, >>>>>> + page); >>>>>> + } >>>>>> + rss[MM_ANONPAGES]++; >>>>>> + VM_BUG_ON(PageAnonExclusive(page)); >>>>>> + } else { >>>>>> + page_dup_file_rmap(page, false); >>>>>> + rss[mm_counter_file(page)]++; >>>>>> + } >>>>>> } >>>>>> - rss[MM_ANONPAGES]++; >>>>>> - } else if (page) { >>>>>> - folio_get(folio); >>>>>> - page_dup_file_rmap(page, false); >>>>>> - rss[mm_counter_file(page)]++; >>>>>> + >>>>>> + nr = i; >>>>>> + folio_ref_add(folio, nr); >>>>>> } >>>>>> /* >>>>>> @@ -968,24 +1051,28 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct >>>>>> vm_area_struct *src_vma, >>>>>> * in the parent and the child >>>>>> */ >>>>>> if (is_cow_mapping(vm_flags) && pte_write(pte)) { >>>>>> - ptep_set_wrprotect(src_mm, addr, src_pte); >>>>>> + ptep_set_wrprotects(src_mm, addr, src_pte, nr); >>>>>> pte = pte_wrprotect(pte); >>>>> >>>>> You likely want an "any_pte_writable" check here instead, no? >>>>> >>>>> Any operations that target a single indiividual PTE while multiple PTEs are >>>>> adjusted are suspicious :) >>>> >>>> The idea is that I've already constrained the batch of pages such that the >>>> permissions are all the same (see folio_nr_pages_cont_mapped()). So if the first >>>> pte is writable, then they all are - something has gone badly wrong if some are >>>> writable and others are not. >>> >>> I wonder if it would be cleaner and easier to not do that, though. >>> >>> Simply record if any pte is writable. Afterwards they will *all* be R/O and you >>> can set the cont bit, correct? >> >> Oh I see what you mean - that only works for cow mappings though. If you have a >> shared mapping, you won't be making it read-only at fork. So if we ignore >> pte_write() state when demarking the batches, we will end up with a batch of >> pages with a mix of RO and RW in the parent, but then we set_ptes() for the >> child and those pages will all have the permissions of the first page of the batch. > > I see what you mean. > > After fork(), all anon pages will be R/O in the parent and the child. > Easy. If any PTE is writable, wrprotect all in the parent and the child. > > After fork(), all shared pages can be R/O or R/W in the parent. For > simplicity, I think you can simply set them all R/O in the child. So if > any PTE is writable, wrprotect all in the child.
Or better: if any is R/O, set them all R/O. Otherwise just leave them as is.
But devil is in the detail.
-- Cheers,
David / dhildenb
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