[PATCH v9 4/8] mm: add a template-based fast path for zone-device page init

["Li Zhe" <lizhe.67@bytedance.com>]

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, as suggested by Muchun. 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%.

Suggested-by: Muchun Song <muchun.song@linux.dev>
Suggested-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 mm/mm_init.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

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);
+	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);
+		}
 
 		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
 			cond_resched();
-- 
2.20.1