Re: [PATCH RFC 09/35] mm/mm_init: make memmap_init_compound() look more like prep_compound_page()

David Hildenbrand
2025-08-22T18:13:31Z

On 22.08.25 17:27, Mike Rapoport wrote:
> On Thu, Aug 21, 2025 at 10:06:35PM +0200, David Hildenbrand wrote:
>> Grepping for "prep_compound_page" leaves on clueless how devdax gets its
>> compound pages initialized.
>>
>> Let's add a comment that might help finding this open-coded
>> prep_compound_page() initialization more easily.
>>
>> Further, let's be less smart about the ordering of initialization and just
>> perform the prep_compound_head() call after all tail pages were
>> initialized: just like prep_compound_page() does.
>>
>> No need for a lengthy comment then: again, just like prep_compound_page().
>>
>> Note that prep_compound_head() already does initialize stuff in page[2]
>> through prep_compound_head() that successive tail page initialization
>> will overwrite: _deferred_list, and on 32bit _entire_mapcount and
>> _pincount. Very likely 32bit does not apply, and likely nobody ever ends
>> up testing whether the _deferred_list is empty.
>>
>> So it shouldn't be a fix at this point, but certainly something to clean
>> up.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/mm_init.c | 13 +++++--------
>> 1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 5c21b3af216b2..708466c5b2cc9 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1091,6 +1091,10 @@ static void __ref memmap_init_compound(struct page *head,
>> unsigned long pfn, end_pfn = head_pfn + nr_pages;
>> unsigned int order = pgmap->vmemmap_shift;
>>
>> + /*
>> + * This is an open-coded prep_compound_page() whereby we avoid
>> + * walking pages twice by initializing them in the same go.
>> + */
>
> While on it, can you also mention that prep_compound_page() is not used to
> properly set page zone link?

Sure, thanks!

--
Cheers

David / dhildenb