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

[David Hildenbrand <david@redhat.com>]

On 04.12.23 11:54, 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. 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             | 195 ++++++++++++++++++++++++++++++----------
>   2 files changed, 162 insertions(+), 46 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..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));
> +
> +	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)]++;
> -	}

This likely looks a lot neater if you keep the existing structure.

For example, you can simply have on the !anon path

} else if (page) {
	folio = page_folio(page);
	nr = folio_nr_pages_cont_mapped ...
	folio_ref_add(folio, nr);
	for (i = 0; i < nr; i++, page++)
		page_dup_file_rmap(page, false);
	rss[mm_counter_file(&folio->page)] += nr;
}

-- 
Cheers,

David / dhildenb


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