[PATCH v9 02/14] mm: move page zone helpers from mm.h to mmzone.h

[Matthew Wilcox]

On Thu, Jun 15, 2023 at 03:33:12PM -0400, Peter Xu wrote:
> My question is whether page_zonenum() is ready for taking all kinds of tail
> pages?
> 
> Zone device tail pages all look fine, per memmap_init_zone_device().  The
> question was other kinds of usual compound pages, like either thp or
> hugetlb.  IIUC page->flags can be uninitialized for those tail pages.

I don't think that's true.  It's my understanding that page->flags is
initialised for all pages in memmap at boot / hotplug / delayed-init
time.  So you can check things like zone, node, etc on literally any
page.  Contrariwise, those flags are not available in tail pages for
use by the entity that has allocated a compound page / large folio.

Also, I don't believe zone device pages support compound allocation.
I think they're always allocated as order-0.