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

Vlastimil Babka
2026-01-16T17:27:44Z

On 1/16/26 18:20, Jason Gunthorpe wrote:
> 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?

Yeah sorry that was imprecise. I meant the struct page/folio layout
implementation details (which may or may not be related to the page allocator).

> Jason
>