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

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 16 Jan 2026 13:49:47 -0400

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