Re: [PATCH v2 1/5] mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> In preparation for FLEXIBLE_THP support, improve
> folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
> passed to it. In this case, all contained pages are accounted using the
> "small" pages scheme.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Yin, Fengwei <fengwei.yin@intel.com>

> ---
>  mm/rmap.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1d8369549424..82ef5ba363d1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1278,31 +1278,43 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
>   * This means the inc-and-test can be bypassed.
>   * The folio does not have to be locked.
>   *
> - * If the folio is large, it is accounted as a THP.  As the folio
> + * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
>   * is new, it's assumed to be mapped exclusively by a single process.
>   */
>  void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>  		unsigned long address)
>  {
> -	int nr;
> +	int nr = folio_nr_pages(folio);
> +	int i;
> +	struct page *page;
>  
> -	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
> +	VM_BUG_ON_VMA(address < vma->vm_start ||
> +			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
>  	__folio_set_swapbacked(folio);
>  
> -	if (likely(!folio_test_pmd_mappable(folio))) {
> +	if (!folio_test_large(folio)) {
>  		/* increment count (starts at -1) */
>  		atomic_set(&folio->_mapcount, 0);
> -		nr = 1;
> +		__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
> +	} else if (!folio_test_pmd_mappable(folio)) {
> +		/* increment count (starts at 0) */
> +		atomic_set(&folio->_nr_pages_mapped, nr);
> +
> +		page = &folio->page;
> +		for (i = 0; i < nr; i++, page++, address += PAGE_SIZE) {
> +			/* increment count (starts at -1) */
> +			atomic_set(&page->_mapcount, 0);
> +			__page_set_anon_rmap(folio, page, vma, address, 1);
> +		}
>  	} else {
>  		/* increment count (starts at -1) */
>  		atomic_set(&folio->_entire_mapcount, 0);
>  		atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
> -		nr = folio_nr_pages(folio);
>  		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
> +		__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>  	}
>  
>  	__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
> -	__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>  }
>  
>  /**