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

Vlastimil Babka
2026-01-16T19:17:33Z

On 1/16/26 18:49, Jason Gunthorpe wrote:
> On Fri, Jan 16, 2026 at 12:10:16PM +0100, Francois Dugast wrote:
>> -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
>
> This seems too weird, why is it in the loop? There is only one
> _nr_pages per folio.

I suppose we could be getting say an order-9 folio that was previously used
as two order-8 folios? And each of them had their _nr_pages in their head
and we can't know that at this point so we have to reset everything?

AFAIU this would not be a problem if the clearing of the previous state was
done upon freeing, as e.g. v4 did, but I think you also argued it meant
processing the pages when freeing and then again at reallocation, so it's
now like this instead?

Or maybe you mean that stray _nr_pages in some tail page from previous
lifetimes can't affect the current lifetime in a wrong way for something
looking at said page? I don't know immediately.

> This is mostly zeroing some memory in the tail pages? Why?
>
> Why can't this use the normal helpers, like memmap_init_compound()?
>
> struct folio *new_folio = page
>
> /* First 4 tail pages are part of struct folio */
> for (i = 4; i < (1UL << order); i++) {
> prep_compound_tail(..)
> }
>
> prep_comound_head(page, order)
> new_folio->_nr_pages = 0
>
> ??
>
> Jason