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Date:   Tue, 18 Jul 2023 11:08:02 +0200
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To:     Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>, Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large
 anon folios
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On 18.07.23 10:58, Ryan Roberts wrote:
> 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@arm.com>
>>>>> Reviewed-by: Yu Zhao <yuzhao@google.com>
>>>>> Reviewed-by: Yin Fengwei <fengwei.yin@intel.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.

Let me try again:

Assume, as I wrote, that we are given a folio that is 
"folio_test_large() && !folio_test_pmd_mappable()". That is, a folio 
that is *not* pmd mappable.

If it's not pmd-mappable, certainly, nr_pmdmapped == 0, and therefore, 
"nr < nr_pmdmapped" will never ever trigger.

The only way to have it added to the deferred split queue is, therefore 
"if (!compound)".

So *for these folios*, we will always pass "compound == false" to make 
that "if (!compound)" succeed.


Does that make sense?

> What change are you suggesting, exactly?

Oh, I never suggested a change (I even gave you my RB). I was just 
thinking out loud.

-- 
Cheers,

David / dhildenb