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

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Date:   Mon, 17 Jul 2023 16:30:00 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large
 anon folios

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@....com>
> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

>  		 */
> -		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.