Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()

[David Hildenbrand <david@redhat.com>]

On 14.07.23 18:17, 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
> order-0 folio (or base page) scheme.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> Reviewed-by: Yu Zhao <yuzhao@google.com>
> Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
> ---
>   mm/rmap.c | 28 +++++++++++++++++++++-------
>   1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0c0d8857dfce..f293d072368a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1278,31 +1278,45 @@ 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);
> 
> -	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)) {

Why remove the "likely" here? The patch itself does not change anything 
about that condition.

>   		/* 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)) {
> +		int i;
> +
> +		for (i = 0; i < nr; i++) {
> +			struct page *page = folio_page(folio, i);
> +
> +			/* increment count (starts at -1) */
> +			atomic_set(&page->_mapcount, 0);
> +			__page_set_anon_rmap(folio, page, vma,
> +					address + (i << PAGE_SHIFT), 1);
> +		}
> +
> +		/* increment count (starts at 0) */

That comment is a bit misleading. We're not talking about a mapcount as 
in the other cases here.

> +		atomic_set(&folio->_nr_pages_mapped, nr);
>   	} 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);
> +		__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>   		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
>   	}
> 

Apart from that, LGTM.

-- 
Cheers,

David / dhildenb