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

[David Hildenbrand <david@redhat.com>]

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".

-- 
Cheers,

David / dhildenb