Re: [PATCH v2 02/14] mm/page_alloc: split prep_compound_page into head and tail subparts

[Joao Martins <joao.m.martins@oracle.com>]

On 7/13/21 1:02 AM, Mike Kravetz wrote:
> On 6/17/21 11:44 AM, Joao Martins wrote:
>> Split the utility function prep_compound_page() into head and tail
>> counterparts, and use them accordingly.
>>
>> This is in preparation for sharing the storage for / deduplicating
>> compound page metadata.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>>  mm/page_alloc.c | 32 +++++++++++++++++++++-----------
>>  1 file changed, 21 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 8836e54721ae..95967ce55829 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -741,24 +741,34 @@ void free_compound_page(struct page *page)
>>  	free_the_page(page, compound_order(page));
>>  }
>>  
>> +static void prep_compound_head(struct page *page, unsigned int order)
>> +{
>> +	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
>> +	set_compound_order(page, order);
>> +	atomic_set(compound_mapcount_ptr(page), -1);
>> +	if (hpage_pincount_available(page))
>> +		atomic_set(compound_pincount_ptr(page), 0);
>> +}
>> +
>> +static void prep_compound_tail(struct page *head, int tail_idx)
>> +{
>> +	struct page *p = head + tail_idx;
>> +
>> +	set_page_count(p, 0);
> 
> When you rebase, you should notice this has been removed from
> prep_compound_page as all tail pages should have zero ref count.
> 
/me nods

>> +	p->mapping = TAIL_MAPPING;
>> +	set_compound_head(p, head);
>> +}
>> +
>>  void prep_compound_page(struct page *page, unsigned int order)
>>  {
>>  	int i;
>>  	int nr_pages = 1 << order;
>>  
>>  	__SetPageHead(page);
>> -	for (i = 1; i < nr_pages; i++) {
>> -		struct page *p = page + i;
>> -		set_page_count(p, 0);
>> -		p->mapping = TAIL_MAPPING;
>> -		set_compound_head(p, page);
>> -	}
>> +	for (i = 1; i < nr_pages; i++)
>> +		prep_compound_tail(page, i);
>>  
>> -	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
>> -	set_compound_order(page, order);
>> -	atomic_set(compound_mapcount_ptr(page), -1);
>> -	if (hpage_pincount_available(page))
>> -		atomic_set(compound_pincount_ptr(page), 0);
>> +	prep_compound_head(page, order);
>>  }
>>  
>>  #ifdef CONFIG_DEBUG_PAGEALLOC
>>
> 
> I'll need something like this for demote hugetlb page fuinctionality
> when the pages being demoted have been optimized for minimal vmemmap
> usage.
> 
> Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
Thanks!