Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork()

[Alistair Popple <apopple@nvidia.com>]

Ryan Roberts <ryan.roberts@arm.com> writes:

<snip>

>  /*
>   * 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..8a87a488950c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -924,68 +924,162 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>  	return 0;
>  }
>  
> +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 enforce_uffd_wp,
> +				      int *dirty_nr, int *writable_nr)
> +{
> +	int floops;
> +	int i;
> +	unsigned long pfn;
> +	bool prot_none;
> +	bool uffd_wp;
> +
> +	if (!folio_test_large(folio))
> +		return 1;
> +
> +	/*
> +	 * Loop either to `end` or to end of folio if its contiguously mapped,
> +	 * whichever is smaller.
> +	 */
> +	floops = (end - addr) >> PAGE_SHIFT;
> +	floops = min_t(int, floops,
> +		       folio_pfn(folio_next(folio)) - page_to_pfn(page));

Much better, thanks for addressing my comments here.

> +
> +	pfn = page_to_pfn(page);
> +	prot_none = pte_protnone(ptent);
> +	uffd_wp = pte_uffd_wp(ptent);
> +
> +	*dirty_nr = !!pte_dirty(ptent);
> +	*writable_nr = !!pte_write(ptent);
> +
> +	pfn++;
> +	pte++;
> +
> +	for (i = 1; i < floops; i++) {
> +		ptent = ptep_get(pte);
> +
> +		if (!pte_present(ptent) || pte_pfn(ptent) != pfn ||
> +		    prot_none != pte_protnone(ptent) ||
> +		    (enforce_uffd_wp && uffd_wp != pte_uffd_wp(ptent)))
> +			break;
> +
> +		if (pte_dirty(ptent))
> +			(*dirty_nr)++;
> +		if (pte_write(ptent))
> +			(*writable_nr)++;
> +
> +		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 = false;
> +	bool enforce_uffd_wp = userfaultfd_wp(dst_vma);
> +	int nr_dirty = !!pte_dirty(pte);
> +	int nr_writable = !!pte_write(pte);
> +	int i, ret;
>  
>  	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, enforce_uffd_wp, &nr_dirty,
> +						&nr_writable);
> +		folio_ref_add(folio, nr);
> +
> +		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. */
> +					folio_ref_sub(folio, nr);
> +					ret = copy_present_page(
> +						dst_vma, src_vma, dst_pte,
> +						src_pte, addr, rss, prealloc,
> +						page);
> +					return ret == 0 ? 1 : ret;
> +				}
> +				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)]++;
> -	}
>  
> -	/*
> -	 * If it's a COW mapping, write protect it both
> -	 * in the parent and the child
> -	 */
> -	if (is_cow_mapping(vm_flags) && pte_write(pte)) {
> -		ptep_set_wrprotect(src_mm, addr, src_pte);
> -		pte = pte_wrprotect(pte);
> +		if (i < nr) {
> +			folio_ref_sub(folio, nr - i);
> +			nr = i;
> +		}
>  	}
> -	VM_BUG_ON(page && folio_test_anon(folio) && PageAnonExclusive(page));
>  
>  	/*
> -	 * If it's a shared mapping, mark it clean in
> -	 * the child
> +	 * If it's a shared mapping, mark it clean and write protected in the
> +	 * child, and rely on a write fault to fix up the permissions. This
> +	 * allows determining batch size without having to consider RO/RW
> +	 * permissions. As an optimization, skip wrprotect if all ptes in the
> +	 * batch have the same permissions.
> +	 *
> +	 * If its a private (CoW) mapping, mark it dirty in the child if _any_
> +	 * of the parent mappings in the block were marked dirty. The contiguous
> +	 * block of mappings are all backed by the same folio, so if any are
> +	 * dirty then the whole folio is dirty. This allows determining batch
> +	 * size without having to consider the dirty bit. Further, write protect
> +	 * it both in the parent and the child so that a future write will cause
> +	 * a CoW. As as an optimization, skip the wrprotect if all the ptes in
> +	 * the batch are already readonly.
>  	 */
> -	if (vm_flags & VM_SHARED)
> +	if (vm_flags & VM_SHARED) {
>  		pte = pte_mkclean(pte);
> -	pte = pte_mkold(pte);
> +		if (nr_writable > 0 && nr_writable < nr)
> +			pte = pte_wrprotect(pte);
> +	} else {
> +		if (nr_dirty)
> +			pte = pte_mkdirty(pte);
> +		if (nr_writable) {
> +			ptep_set_wrprotects(src_mm, addr, src_pte, nr);
> +			pte = pte_wrprotect(pte);
> +		}
> +	}
>  
> -	if (!userfaultfd_wp(dst_vma))
> +	pte = pte_mkold(pte);
> +	pte = pte_clear_soft_dirty(pte);
> +	if (!enforce_uffd_wp)
>  		pte = pte_clear_uffd_wp(pte);
>  
> -	set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
> -	return 0;
> +	set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
> +	return nr;

I don't have any further comments and you have addressed my previous
ones so feel free to add:

Reviewed-by: Alistair Popple <apopple@nvidia.com>

However whilst I think the above CoW sequence looks correct it would be
nice if someone else could take a look as well.

>  }
>  
>  static inline struct folio *page_copy_prealloc(struct mm_struct *src_mm,
> @@ -1021,6 +1115,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  	int rss[NR_MM_COUNTERS];
>  	swp_entry_t entry = (swp_entry_t){0};
>  	struct folio *prealloc = NULL;
> +	int nr_ptes;
>  
>  again:
>  	progress = 0;
> @@ -1051,6 +1146,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  	arch_enter_lazy_mmu_mode();
>  
>  	do {
> +		nr_ptes = 1;
> +
>  		/*
>  		 * We are holding two locks at this point - either of them
>  		 * could generate latencies in another task on another CPU.
> @@ -1086,16 +1183,21 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  			 * the now present pte.
>  			 */
>  			WARN_ON_ONCE(ret != -ENOENT);
> +			ret = 0;
>  		}
> -		/* copy_present_pte() will clear `*prealloc' if consumed */
> -		ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte,
> -				       addr, rss, &prealloc);
> +		/* copy_present_ptes() will clear `*prealloc' if consumed */
> +		nr_ptes = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte,
> +					    addr, end, rss, &prealloc);
> +
>  		/*
>  		 * If we need a pre-allocated page for this pte, drop the
>  		 * locks, allocate, and try again.
>  		 */
> -		if (unlikely(ret == -EAGAIN))
> +		if (unlikely(nr_ptes == -EAGAIN)) {
> +			ret = -EAGAIN;
>  			break;
> +		}
> +
>  		if (unlikely(prealloc)) {
>  			/*
>  			 * pre-alloc page cannot be reused by next time so as
> @@ -1106,8 +1208,9 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  			folio_put(prealloc);
>  			prealloc = NULL;
>  		}
> -		progress += 8;
> -	} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
> +		progress += 8 * nr_ptes;
> +	} while (dst_pte += nr_ptes, src_pte += nr_ptes,
> +		 addr += PAGE_SIZE * nr_ptes, addr != end);
>  
>  	arch_leave_lazy_mmu_mode();
>  	pte_unmap_unlock(orig_src_pte, src_ptl);