Messages in this thread | | | Date | Thu, 16 Nov 2023 15:13:14 +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 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.
Why? in the default case, fork() does not even care about MAP_SHARED mappings; it does not copy the page tables/ptes. See vma_needs_copy().
Only in corner cases (e.g., uffd-wp, VM_PFNMAP, VM_MIXEDMAP), or in MAP_PRIVATE mappings, you can even end up in that code.
In MAP_PRIVATE mappings, only anon pages can be R/W, other pages can never be writable, so it does not matter. In VM_PFNMAP/VM_MIXEDMAP likely all permissions match either way.
So you might just wrprotect the !anon pages R/O for the child and nobody should really notice it, write faults will resolve it.
Famous last words :)
-- Cheers,
David / dhildenb
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