Re: [PATCH v3 09/14] mm/page_alloc: reuse tail struct pages for compound pagemaps

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

On 7/28/21 8:28 AM, Dan Williams wrote:
> On Wed, Jul 14, 2021 at 12:36 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>>
>> +       /*
>> +        * With compound page geometry and when struct pages are stored in ram
>> +        * (!altmap) 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.
>> +        * See vmemmap_populate_compound_pages().
>> +        */
>> +       if (!altmap)
>> +               nr_pages = min_t(unsigned long, nr_pages,
> 
> What's the scenario where nr_pages is < 128? Shouldn't alignment
> already be guaranteed?
> 
Oh yeah, that's right.

>> +                                2 * (PAGE_SIZE/sizeof(struct page)));
> 
> 
>> +
>>         __SetPageHead(page);
>>
>>         for (i = 1; i < nr_pages; i++) {
>> @@ -6657,7 +6669,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
>>                         continue;
>>
>>                 memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
>> -                                    pfns_per_compound);
>> +                                    altmap, pfns_per_compound);
> 
> This feels odd, memmap_init_compound() doesn't really care about
> altmap, what do you think about explicitly calculating the parameters
> that memmap_init_compound() needs and passing them in?
> 
> Not a strong requirement to change, but take another look at let me know.
> 

Yeah, memmap_init_compound() indeed doesn't care about @altmap itself -- but a previous
comment was to abstract this away in memmap_init_compound() given the mix of complexity in
memmap_init_zone_device() PAGE_SIZE geometry case and the compound case:

https://lore.kernel.org/linux-mm/CAPcyv4gtSqfmuAaX9cs63OvLkf-h4B_5fPiEnM9p9cqLZztXpg@mail.gmail.com/

Before this was called @ntails above and I hide that calculation in memmap_init_compound().

But I can move this back to the caller:

memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
	(!altmap ? 2 * (PAGE_SIZE/sizeof(struct page))) : pfns_per_compound);

Or with another helper like:

#define compound_nr_pages(__altmap, __nr_pages) \
		(!__altmap ? 2 * (PAGE_SIZE/sizeof(struct page))) : __nr_pages);
			
memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
		     compound_nr_pages(altmap, pfns_per_compound));