Messages in this thread | | | Date | Thu, 16 Nov 2023 14:20:58 +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 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?
-- Cheers,
David / dhildenb
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