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

Vlastimil Babka
2026-01-22T08:02:18Z

On 1/16/26 17:07, 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.

For doing this as a hotfix for 6.19, assuming we'll refactor later:

Acked-by: Vlastimil Babka <vbabka@suse.cz>