[PATCH v6 1/5] mm/zone_device: Reinitialize large zone device private folios

[Jason Gunthorpe]

On Fri, Jan 16, 2026 at 05:07:09PM +0100, Vlastimil Babka wrote:
> On 1/16/26 12:10, Francois Dugast wrote:
> > From: Matthew Brost <matthew.brost at intel.com>
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 63c6ab4fdf08..ac7be07e3361 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -477,10 +477,43 @@ void free_zone_device_folio(struct folio *folio)
> >  	}
> >  }
> >  
> > -void zone_device_page_init(struct page *page, unsigned int order)
> > +void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
> > +			   unsigned int order)
> >  {
> > +	struct page *new_page = page;
> > +	unsigned int i;
> > +
> >  	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >  
> > +	for (i = 0; i < (1UL << order); ++i, ++new_page) {
> > +		struct folio *new_folio = (struct folio *)new_page;
> > +
> > +		/*
> > +		 * new_page could have been part of previous higher order folio
> > +		 * which encodes the order, in page + 1, in the flags bits. We
> > +		 * blindly clear bits which could have set my order field here,
> > +		 * including page head.
> > +		 */
> > +		new_page->flags.f &= ~0xffUL;	/* Clear possible order, page head */
> > +
> > +#ifdef NR_PAGES_IN_LARGE_FOLIO
> > +		/*
> > +		 * This pointer math looks odd, but new_page could have been
> > +		 * part of a previous higher order folio, which sets _nr_pages
> > +		 * in page + 1 (new_page). Therefore, we use pointer casting to
> > +		 * correctly locate the _nr_pages bits within new_page which
> > +		 * could have modified by previous higher order folio.
> > +		 */
> > +		((struct folio *)(new_page - 1))->_nr_pages = 0;
> > +#endif
> > +
> > +		new_folio->mapping = NULL;
> > +		new_folio->pgmap = pgmap;	/* Also clear compound head */
> > +		new_folio->share = 0;   /* fsdax only, unused for device private */
> > +		VM_WARN_ON_FOLIO(folio_ref_count(new_folio), new_folio);
> > +		VM_WARN_ON_FOLIO(!folio_is_zone_device(new_folio), new_folio);
> > +	}
> > +
> >  	/*
> >  	 * Drivers shouldn't be allocating pages after calling
> >  	 * memunmap_pages().
> 
> Can't say I'm a fan of this. It probably works now (so I'm not nacking) but
> seems rather fragile. It seems likely to me somebody will try to change some
> implementation detail in the page allocator and not notice it breaks this,
> for example. I hope we can eventually get to something more robust.

These pages shouldn't be in the buddy allocator at all? The driver
using the ZONE_DEVICE pages is responsible to provide its own
allocator.

Did you mean something else?

Jason