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

Jason Gunthorpe
2026-01-16T17:50:23Z

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.

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