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

On 7/15/2021 5:52 AM, Joao Martins wrote:

quoted

quoted

+               __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