Re: [PATCH v5 4/8] mm: add a template-based fast path for zone-device page init

[Mike Rapoport <rppt@kernel.org>]

On Wed, Jul 01, 2026 at 05:05:49PM +0800, Li Zhe wrote:
> memmap_init_zone_device() repeats nearly identical head-page
> initialization for each PFN. Prepare one reusable ZONE_DEVICE head-page
> template through the existing slow path, refresh the PFN-dependent
> fields in that template before each copy, and memcpy it into each
> destination page.
> 
> This reduces the average rebind time from 244.28 ms to 217.19 ms, or
> about 11%.
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> ---
>  mm/mm_init.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 74 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 4c7fad440c2a..cc8417951467 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1066,6 +1066,50 @@ static void __ref zone_device_page_init_slow(struct page *page,
>  		set_page_count(page, 0);
>  }
>  
> +static inline bool zone_device_page_init_optimization_enabled(void)
> +{
> +	/*
> +	 * The template fast path copies a preinitialized struct page image.
> +	 * Skip it when the page_ref_set tracepoint is enabled.
> +	 */
> +	return !page_ref_tracepoint_active(page_ref_set);
> +}
> +
> +static inline void zone_device_template_page_init(struct page *template,
> +						  struct page *src)
> +{
> +	memcpy(template, src, sizeof(*template));
> +}
> +
> +/*
> + * 'template' is a reusable page prototype rather than a strictly immutable
> + * object. Most ZONE_DEVICE fields stay constant across the pages covered by
> + * the current template, but section bits and page->virtual may still depend
> + * on the PFN. Refresh those PFN-dependent fields in the template before
> + * copying it into @page.
> + */
> +static inline void zone_device_page_update_template(struct page *template,
> +		unsigned long pfn)
> +{
> +	set_page_section_from_pfn(template, pfn);
> +#ifdef WANT_PAGE_VIRTUAL
> +	if (!is_highmem_idx(ZONE_DEVICE))
> +		set_page_address(template, __va(pfn << PAGE_SHIFT));
> +#endif
> +}
> +
> +static void zone_device_page_init_from_template(struct page *page,
> +		unsigned long pfn, struct page *template)
> +{
> +	/*
> +	 * 'template' carries the invariant portion of a ZONE_DEVICE struct
> +	 * page. Update the PFN-dependent fields in place before copying it
> +	 * to the destination page.
> +	 */
> +	zone_device_page_update_template(template, pfn);
> +	memcpy(page, template, sizeof(*page));
> +}
> +

The whole bunch of template functions look like it could be useful for
initialization of the non-zone-device struct pages as well.

As I mentioned previously, it's interesting to see if this approach speeds
up normal memory map initialization as well. If if does could have a single
set of the template functions.

-- 
Sincerely yours,
Mike.