Re: [PATCH v3 04/14] mm/memremap: add ZONE_DEVICE support for compound pages

[Jane Chu <jane.chu@oracle.com>]

On 7/15/2021 5:52 AM, Joao Martins wrote:
>>> +               __init_zone_device_page(page + i, pfn + i, zone_idx,
>>> +                                       nid, pgmap);
>>> +               prep_compound_tail(page, i);
>>> +
>>> +               /*
>>> +                * The first and second tail pages need to
>>> +                * initialized first, hence the head page is
>>> +                * prepared last.
>> I'd change this comment to say why rather than restate what can be
>> gleaned from the code. It's actually not clear to me why this order is
>> necessary.
>>
> So the first tail page stores mapcount_ptr and compound order, and the
> second tail page stores pincount_ptr. prep_compound_head() does this:
> 
> 	set_compound_order(page, order);
> 	atomic_set(compound_mapcount_ptr(page), -1);
> 	if (hpage_pincount_available(page))
> 		atomic_set(compound_pincount_ptr(page), 0);
> 
> So we need those tail pages initialized first prior to initializing the head.
> 
> I can expand the comment above to make it clear why we need first and second tail pages.
> 

Perhaps just say
   The reason prep_compound_head() is called after the 1st and 2nd tail
   pages have been initialized is: so it overwrites some of the tail page
   fields setup by __init_zone_device_page(), rather than the other way 
around.
?

thanks,
-jane