Re: [PATCH v2 14/15] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()

[Ryan Roberts]

On 25/01/2024 19:32, David Hildenbrand wrote:
> Let's always ignore the accessed/young bit: we'll always mark the PTE
> as old in our child process during fork, and upcoming users will
> similarly not care.
> 
> Ignore the dirty bit only if we don't want to duplicate the dirty bit
> into the child process during fork. Maybe, we could just set all PTEs
> in the child dirty if any PTE is dirty. For now, let's keep the behavior
> unchanged, this can be optimized later if required.
> 
> Ignore the soft-dirty bit only if the bit doesn't have any meaning in
> the src vma, and similarly won't have any in the copied dst vma.
> 
> For now, we won't bother with the uffd-wp bit.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

> ---
>  mm/memory.c | 36 +++++++++++++++++++++++++++++++-----
>  1 file changed, 31 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4d1be89a01ee0..b3f035fe54c8d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -953,24 +953,44 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
>  	set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>  }
>  
> +/* Flags for folio_pte_batch(). */
> +typedef int __bitwise fpb_t;
> +
> +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
> +#define FPB_IGNORE_DIRTY		((__force fpb_t)BIT(0))
> +
> +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
> +#define FPB_IGNORE_SOFT_DIRTY		((__force fpb_t)BIT(1))
> +
> +static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> +{
> +	if (flags & FPB_IGNORE_DIRTY)
> +		pte = pte_mkclean(pte);
> +	if (likely(flags & FPB_IGNORE_SOFT_DIRTY))
> +		pte = pte_clear_soft_dirty(pte);
> +	return pte_mkold(pte);
> +}
> +
>  /*
>   * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>   * pages of the same folio.
>   *
> - * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
> + * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
> + * the accessed bit, dirty bit (with FPB_IGNORE_DIRTY) and soft-dirty bit
> + * (with FPB_IGNORE_SOFT_DIRTY).
>   */
>  static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> -		pte_t *start_ptep, pte_t pte, int max_nr)
> +		pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags)
>  {
>  	unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
>  	const pte_t *end_ptep = start_ptep + max_nr;
> -	pte_t expected_pte = pte_next_pfn(pte);
> +	pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte), flags);
>  	pte_t *ptep = start_ptep + 1;
>  
>  	VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>  
>  	while (ptep != end_ptep) {
> -		pte = ptep_get(ptep);
> +		pte = __pte_batch_clear_ignored(ptep_get(ptep), flags);
>  
>  		if (!pte_same(pte, expected_pte))
>  			break;
> @@ -1004,6 +1024,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>  {
>  	struct page *page;
>  	struct folio *folio;
> +	fpb_t flags = 0;
>  	int err, nr;
>  
>  	page = vm_normal_page(src_vma, addr, pte);
> @@ -1018,7 +1039,12 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>  	 * by keeping the batching logic separate.
>  	 */
>  	if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) {
> -		nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr);
> +		if (src_vma->vm_flags & VM_SHARED)
> +			flags |= FPB_IGNORE_DIRTY;
> +		if (!vma_soft_dirty_enabled(src_vma))
> +			flags |= FPB_IGNORE_SOFT_DIRTY;
> +
> +		nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags);
>  		folio_ref_add(folio, nr);
>  		if (folio_test_anon(folio)) {
>  			if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,