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

[David Hildenbrand]

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@xxxxxxx>
> ---
>   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);

You're changing the order of mapcount vs. refcount increment. Don't. 
Make sure your refcount >= mapcount.

You can do that easily by doing the folio_ref_add(folio, nr) first and 
then decrementing in case of error accordingly. Errors due to pinned 
pages are the corner case.

I'll note that it will make a lot of sense to have batch variants of 
page_try_dup_anon_rmap() and page_dup_file_rmap().

Especially, the batch variant of page_try_dup_anon_rmap() would only 
check once if the folio maybe pinned, and in that case, you can simply 
drop all references again. So you either have all or no ptes to process, 
which makes that code easier.

But that can be added on top, and I'll happily do that.

--
Cheers,

David / dhildenb