Hi,
> 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.
>
> 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(v7.2-rc1):
> Average of rebinds for nd_pmem driver: 244.28 ms
>
> With this patch and its prerequisites applied:
> Average of rebinds for nd_pmem driver: 215.55 ms
>
> This reduces the average memmap initialization time measured during rebind
> from 244.28 ms to 215.55 ms, or about 11%.
>
> Signed-off-by: Li Zhe <[email protected]>
>
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index a70acb7431a6..56a36a71ba89 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1065,6 +1065,35 @@ static void __ref zone_device_page_init_slow(struct
> page *page,
> set_page_count(page, 0);
> }
>
> +/*
> + * '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);
Looks like it's the only user of zone_device_page_update_template().
I'd just fold it here and drop the comment.
> + memcpy(page, template, sizeof(*page));
> +}
> +
> /*
> * With compound page geometry and when struct pages are stored in ram most
> * tail pages are reused. Consequently, the amount of unique struct pages to
> @@ -1127,6 +1156,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
> unsigned long zone_idx = zone_idx(zone);
> unsigned long start = jiffies;
> int nid = pgdat->node_id;
> + struct page template;
>
> if (WARN_ON_ONCE(!pgmap || zone_idx != ZONE_DEVICE))
> return;
> @@ -1144,7 +1174,22 @@ void __ref memmap_init_zone_device(struct zone *zone,
> for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
> struct page *page = pfn_to_page(pfn);
>
> - zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
> + if (pfn == start_pfn) {
> + /*
> + * Seed the reusable head-page template from the
> + * first real struct page. This initializes the
> + * first page through the existing slow path and
> + * then reuses that final state as the template
> + * for subsequent pages.
> + */
> + zone_device_page_init_slow(page, pfn, zone_idx,
> + nid, pgmap);
> + /* init template page */
> + memcpy(&template, page, sizeof(*page));
> + } else {
> + zone_device_page_init_from_template(page, pfn,
> + &template);
> + }
Why can't we init the template for the first page being initialized and
then call zone_device_page_init_from_template() unconditionally?
The same applies to the tail pages initialization.
--
Sincerely yours,
Mike.
