Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large anon folios

[Matthew Wilcox]

On Mon, Jul 17, 2023 at 03:31:08PM +0100, Ryan Roberts wrote:
> In preparation for 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>

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

>  		 */
> -		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);

I wonder if it's worth introducing a folio_test_deferred_split() (better
naming appreciated ...) to allow us to allocate order-1 folios and not
do horrible things.  Maybe it's not worth supporting order-1 folios;
we're always better off going to order-2 immediately.  Just thinking.