[PATCH v5 5/8] mm: extend the template fast path to zone-device compound tails

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

The template fast path from the previous patch only accelerates head
pages. Compound tails in memmap_init_compound() still go through the
slow path one by one.

Build separate head and tail templates and reuse one prepared tail
template across the tail pages in a compound range. Head pages preserve
the existing refcount policy, while compound tails always start with a
refcount of 0 after prep_compound_tail().

This extends the template-copy fast path to pfns_per_compound > 1
without changing the existing slow path. Tail-page PFN-dependent fields
are refreshed in the reusable tail template before each copy.

Tested in a VM with a 100 GB devdax namespace (align=2097152) on Intel
Ice Lake server. This test exercises the dax_pmem rebind path and
measures memmap initialization latency.

Test procedure:
Unbind and rebind the dax_pmem driver 30 times, collect memmap
initialization time from the pr_debug() output of memmap_init_zone_device().

Base(v7.2-rc1):
  First binding: 1462 ms
  Average of subsequent rebinds: 273.31 ms

With this patch and its prerequisites applied:
  First binding: 1403 ms
  Average of subsequent rebinds: 244.37 ms

This reduces the average rebind time from 273.31 ms to 244.37 ms, or
about 10.6%.

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 mm/mm_init.c | 47 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index cc8417951467..60794050bc07 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1081,6 +1081,16 @@ static inline void zone_device_template_page_init(struct page *template,
 	memcpy(template, src, sizeof(*template));
 }
 
+static inline void zone_device_tail_page_init(struct page *page,
+		unsigned long pfn, unsigned long zone_idx, int nid,
+		struct dev_pagemap *pgmap, const struct page *head,
+		unsigned int order)
+{
+	zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
+	prep_compound_tail(page, head, order);
+	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
@@ -1138,10 +1148,12 @@ static void __ref memmap_init_compound(struct page *head,
 				       unsigned long head_pfn,
 				       unsigned long zone_idx, int nid,
 				       struct dev_pagemap *pgmap,
-				       unsigned long nr_pages)
+				       unsigned long nr_pages,
+				       bool use_template)
 {
 	unsigned long pfn, end_pfn = head_pfn + nr_pages;
 	unsigned int order = pgmap->vmemmap_shift;
+	struct page template;
 
 	/*
 	 * We have to initialize the pages, including setting up page links.
@@ -1150,12 +1162,31 @@ static void __ref memmap_init_compound(struct page *head,
 	 * the pages in the same go.
 	 */
 	__SetPageHead(head);
-	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
+
+	pfn = head_pfn + 1;
+	/*
+	 * All tails of the same compound page share the state established by
+	 * prep_compound_tail(). Reuse one tail template for the whole range and
+	 * refresh only the PFN-dependent fields in that template before each copy.
+	 */
+	if (use_template) {
 		struct page *page = pfn_to_page(pfn);
 
-		zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
-		prep_compound_tail(page, head, order);
-		set_page_count(page, 0);
+		zone_device_tail_page_init(page, pfn, zone_idx, nid,
+					   pgmap, head, order);
+		zone_device_template_page_init(&template, page);
+		pfn++;
+	}
+
+	for (; pfn < end_pfn; pfn++) {
+		struct page *page = pfn_to_page(pfn);
+
+		if (use_template)
+			zone_device_page_init_from_template(page, pfn,
+							    &template);
+		else
+			zone_device_tail_page_init(page, pfn, zone_idx, nid,
+						   pgmap, head, order);
 	}
 	prep_compound_head(head, order);
 }
@@ -1205,7 +1236,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
 		zone_device_template_page_init(&template, page);
 		if (pfns_per_compound != 1)
 			memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				compound_nr_pages(start_pfn, altmap, pgmap));
+				compound_nr_pages(start_pfn, altmap, pgmap),
+				use_template);
 		pfn += pfns_per_compound;
 	}
 
@@ -1226,7 +1258,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
 			continue;
 
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     compound_nr_pages(pfn, altmap, pgmap));
+				     compound_nr_pages(pfn, altmap, pgmap),
+				     use_template);
 	}
 
 	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE);
-- 
2.20.1