Re: [PATCH v2 2/5] mm: Allow deferred splitting of arbitrary large anon folios

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> writes:

> With the introduction of large folios for anonymous memory, we would
> like to be able to split them when they have unmapped subpages, in order
> to free those unused pages under memory pressure. So remove the
> artificial requirement that the large folio needed to be at least
> PMD-sized.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> ---
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 82ef5ba363d1..bbcb2308a1c5 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1474,7 +1474,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>  		 * page of the folio is unmapped and at least one page
>  		 * is still mapped.
>  		 */
> -		if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
> +		if (folio_test_large(folio) && folio_test_anon(folio))
>  			if (!compound || nr < nr_pmdmapped)
>  				deferred_split_folio(folio);
>  	}

One possible issue is that even for large folios mapped only in one
process, in zap_pte_range(), we will always call deferred_split_folio()
unnecessarily before freeing a large folio.

Best Regards,
Huang, Ying