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 17:01:23 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>
Cc:     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 16:42, David Hildenbrand wrote:
> On 17.07.23 16:31, 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>
>> ---
>>   mm/rmap.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 0c0d8857dfce..2baf57d65c23 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1430,7 +1430,7 @@ 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);
> 
> !compound will always be true I guess, so nr_pmdmapped == 0 (which will always
> be the case) will be ignored.

I don't follow why !compound will always be true. This function is
page_remove_rmap() (not folio_remove_rmap_range() which I add in a later patch).
page_remove_rmap() can work on pmd-mapped pages where compound=true is passed in.

> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
>