[PATCH RFC 4/9] mm/page_alloc: Reuse tail struct pages for compound pagemaps

[Joao Martins <joao.m.martins@oracle.com>]

When PGMAP_COMPOUND is set, all pages are onlined at a given huge page
alignment and using compound pages to describe them as opposed to a
struct per 4K.

To minimize struct page overhead and given the usage of compound pages we
utilize the fact that most tail pages look the same, we online the
subsection while pointing to the same pages. Thus request VMEMMAP_REUSE
in add_pages.

With VMEMMAP_REUSE, provided we reuse most tail pages the amount of
struct pages we need to initialize is a lot smaller that the total
amount of structs we would normnally online. Thus allow an @init_order
to be passed to specify how much pages we want to prep upon creating a
compound page.

Finally when onlining all struct pages in memmap_init_zone_device, make
sure that we only initialize the unique struct pages i.e. the first 2
4K pages from @align which means 128 struct pages out of 32768 for 2M
@align or 262144 for a 1G @align.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
---
 mm/memremap.c   |  4 +++-
 mm/page_alloc.c | 23 ++++++++++++++++++++---
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/mm/memremap.c b/mm/memremap.c
index ecfa74848ac6..3eca07916b9d 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -253,8 +253,10 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
 			goto err_kasan;
 		}
 
-		if (pgmap->flags & PGMAP_COMPOUND)
+		if (pgmap->flags & PGMAP_COMPOUND) {
 			params->align = pgmap->align;
+			params->flags = MEMHP_REUSE_VMEMMAP;
+		}
 
 		error = arch_add_memory(nid, range->start, range_len(range),
 					params);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9716ecd58e29..180a7d4e9285 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -691,10 +691,11 @@ void free_compound_page(struct page *page)
 	__free_pages_ok(page, compound_order(page), FPI_NONE);
 }
 
-void prep_compound_page(struct page *page, unsigned int order)
+static void __prep_compound_page(struct page *page, unsigned int order,
+				 unsigned int init_order)
 {
 	int i;
-	int nr_pages = 1 << order;
+	int nr_pages = 1 << init_order;
 
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++) {
@@ -711,6 +712,11 @@ void prep_compound_page(struct page *page, unsigned int order)
 		atomic_set(compound_pincount_ptr(page), 0);
 }
 
+void prep_compound_page(struct page *page, unsigned int order)
+{
+	__prep_compound_page(page, order, order);
+}
+
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
 
@@ -6108,6 +6114,9 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
 }
 
 #ifdef CONFIG_ZONE_DEVICE
+
+#define MEMMAP_COMPOUND_SIZE (2 * (PAGE_SIZE/sizeof(struct page)))
+
 void __ref memmap_init_zone_device(struct zone *zone,
 				   unsigned long start_pfn,
 				   unsigned long nr_pages,
@@ -6138,6 +6147,12 @@ void __ref memmap_init_zone_device(struct zone *zone,
 	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 
+		/* Skip already initialized pages. */
+		if (compound && (pfn % align >= MEMMAP_COMPOUND_SIZE)) {
+			pfn = ALIGN(pfn, align) - 1;
+			continue;
+		}
+
 		__init_single_page(page, pfn, zone_idx, nid);
 
 		/*
@@ -6175,7 +6190,9 @@ void __ref memmap_init_zone_device(struct zone *zone,
 
 	if (compound) {
 		for (pfn = start_pfn; pfn < end_pfn; pfn += align)
-			prep_compound_page(pfn_to_page(pfn), order_base_2(align));
+			__prep_compound_page(pfn_to_page(pfn),
+					   order_base_2(align),
+					   order_base_2(MEMMAP_COMPOUND_SIZE));
 	}
 
 	pr_info("%s initialised %lu pages in %ums\n", __func__,
-- 
2.17.1
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