Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

David Hildenbrand
2023-11-16T11:03:46Z

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 :)

--
Cheers,

David / dhildenb