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

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Message-ID: <7319a1aa-7c72-82e9-f26d-eeccb6fdf35b@arm.com>
Date:   Tue, 18 Jul 2023 09:58:04 +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 17:48, David Hildenbrand wrote:
> On 17.07.23 18:01, Ryan Roberts wrote:
>> 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.
> 
> I was talking about the folio_test_pmd_mappable() -> folio_test_large() change.
> For folio_test_large() && !folio_test_pmd_mappable() I expect that we'll never
> pass in "compound=true".
> 

Sorry David, I've been staring at the code and your comment, and I still don't
understand your point. I assumed you were trying to say that compound is always
false and therefore "if (!compound || nr < nr_pmdmapped)" can be removed? But
its not the case that compound is always false; it will be true when called to
remove a pmd-mapped compound page. What change are you suggesting, exactly?