Re: [PATCH v11 3/7] mm: add a template-based fast path for zone-device page init

[Mike Rapoport <rppt@kernel.org>]

On Mon, Aug 31, 2026 at 07:16:34PM +0800, Li Zhe wrote:
> memmap_init_zone_device() repeats nearly identical head-page
> initialization for each PFN. Initialize the first real ZONE_DEVICE head
> page through the existing path, copy that final state into a reusable
> template, refresh the PFN-dependent fields in that template before each
> copy, and copy it into the remaining destination pages.
> 
> Use the template path unconditionally. The page_ref_set tracepoint is
> primarily a debugging aid, while this code is still initializing struct
> pages before they are handed out. From the perspective of users of those
> pages, the initialization-time refcount transitions are not part of the
> observable page lifetime.
> 
> This means page_ref_set will no longer observe every initialization-time
> refcount assignment for copied ZONE_DEVICE head pages. The impact is
> controlled because the final initialized struct page state is unchanged,
> and keeping a separate non-template path only for this local tracepoint
> observability would add complexity to the common path.
> 
> This patch accelerates head-page initialization. The pfns_per_compound
> == 1 case gets the full benefit here, compound tails are handled in the
> next patch.
> 
> Tested in a VM with a 100 GB fsdax namespace device configured with
> map=dev on Intel Ice Lake server. This test exercises the nd_pmem rebind
> path (pfns_per_compound == 1).
> 
> Test procedure:
> Rebind the nd_pmem driver 30 times and collect the memmap initialization
> time from the pr_debug() output of memmap_init_zone_device().
> 
> Base(v7.3-rc1):
>   Average of rebinds for nd_pmem driver: 221.07 ms
> 
> With this patch and its prerequisites applied:
>   Average of rebinds for nd_pmem driver: 155.00 ms
> 
> This reduces the average memmap initialization time measured during
> rebind from 221.07 ms to 155.00 ms, or about 29.9%.
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>

With one nit below

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/mm_init.c | 38 +++++++++++++++++++++++++++++++++++---
>  1 file changed, 35 insertions(+), 3 deletions(-)
> 
> @@ -1093,10 +1106,29 @@ void __ref memmap_init_zone_device(struct zone *zone,
>  		nr_pages = end_pfn - start_pfn;
>  	}
>  
> -	for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
> -		struct page *page = pfn_to_page(pfn);
> +	if (!nr_pages)
> +		return;
>  
> -		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
> +	/*
> +	 * Seed the reusable head-page template from the first real struct
> +	 * page. The normal page-init and refcount helpers must operate on
> +	 * a real memmap entry rather than a stack object.
> +	 */
> +	pfn = start_pfn;
> +	page = pfn_to_page(pfn);
> +	__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
> +	memcpy(&template, page, sizeof(*page));
> +	if (pfns_per_compound != 1)
> +		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
> +				     compound_nr_pages(pfn, altmap, pgmap));
> +	pfn += pfns_per_compound;
> +
> +	/* Initialize the remaining head pages from template. */
> +	for (; pfn < end_pfn; pfn += pfns_per_compound) {
> +		page = pfn_to_page(pfn);
> +
> +		zone_device_page_init_from_template(page, pfn,
> +						    &template);

Nit: this seems to fit into 80-chars line without wrapping.

>  
>  		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
>  			cond_resched();
> -- 
> 2.20.1

-- 
Sincerely yours,
Mike.