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

[Ryan Roberts <ryan.roberts@arm.com>]

On 18/07/2023 10:08, David Hildenbrand wrote:
> 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?

Yes I agree with all of this. I thought you were pointing out an issue or
proposing a change to the logic. Hence my confusion.

> 
>> What change are you suggesting, exactly?
> 
> Oh, I never suggested a change (I even gave you my RB). I was just thinking out
> loud.
>