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 16:41:34 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Matthew Wilcox <willy@...radead.org>
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 17/07/2023 16:30, Matthew Wilcox wrote:
> 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>

Thanks!

> 
>>  		 */
>> -		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.

There is more than just _deferred_list in the 3rd page; you also have _flags_2a
and _head_2a. I guess you know much better than me what they store. But I'm
guessing its harder than jsut not splitting an order-1 page?

With the direction of large anon folios (_not_ retrying with every order down to
0), I'm not sure what the use case would be for order-1 anyway?