On 7/22/21 1:38 AM, Jane Chu wrote:
>
> 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.
> ?
Yeap, something along those lines is what I was thinking. Perhaps explicitly
mentioning
the struct page fields that 1st and 2nd tail pages store to avoid the reader
thinking it's
arbitrarily picked.
Joao
