Re: [PATCH v6 5/5] mm/page_alloc: reuse tail struct pages for compound devmaps

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

On 2/24/22 05:57, Muchun Song wrote:
> On Thu, Feb 24, 2022 at 3:48 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6653,6 +6653,20 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>>         }
>>  }
>>
>> +/*
>> + * With compound page geometry and when struct pages are stored in ram most
>> + * tail pages are reused. Consequently, the amount of unique struct pages to
>> + * initialize is a lot smaller that the total amount of struct pages being
>> + * mapped. This is a paired / mild layering violation with explicit knowledge
>> + * of how the sparse_vmemmap internals handle compound pages in the lack
>> + * of an altmap. See vmemmap_populate_compound_pages().
>> + */
>> +static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
>> +                                             unsigned long nr_pages)
>> +{
>> +       return !altmap ? 2 * (PAGE_SIZE/sizeof(struct page)) : nr_pages;
>> +}
>> +
> 
> Should be:
> 
> return is_power_of_2(sizeof(struct page)) &&
>        !altmap ? 2 * (PAGE_SIZE/sizeof(struct page)) : nr_pages;
> 
I only half-address your previous comment.

I've fixed it now.