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

Jason Gunthorpe
2026-01-16T17:21:00Z

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@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