[PATCH v3 09/14] mm/page_alloc: reuse tail struct pages for compound pagemaps

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

Currently memmap_init_zone_device() ends up initializing 32768 pages
when it only needs to initialize 128 given tail page reuse. That
number is worse with 1GB compound page geometries, 262144 instead of
128. Update memmap_init_zone_device() to skip redundant
initialization, detailed below.

When a pgmap @geometry is set, all pages are mapped at a given huge page
alignment and use compound pages to describe them as opposed to a
struct per 4K.

With @geometry > PAGE_SIZE and when struct pages are stored in ram
(!altmap) most tail pages are reused. Consequently, the amount of unique
struct pages is a lot smaller that the total amount of struct pages
being mapped.

The altmap path is left alone since it does not support memory savings
based on compound pagemap geometries.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
---
 mm/page_alloc.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 188cb5f8c308..96975edac0a8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6600,11 +6600,23 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
 static void __ref memmap_init_compound(struct page *page, unsigned long pfn,
 					unsigned long zone_idx, int nid,
 					struct dev_pagemap *pgmap,
+					struct vmem_altmap *altmap,
 					unsigned long nr_pages)
 {
 	unsigned int order_align = order_base_2(nr_pages);
 	unsigned long i;
 
+	/*
+	 * With compound page geometry and when struct pages are stored in ram
+	 * (!altmap) most tail pages are reused. Consequently, the amount of
+	 * unique struct pages to initialize is a lot smaller that the total
+	 * amount of struct pages being mapped.
+	 * See vmemmap_populate_compound_pages().
+	 */
+	if (!altmap)
+		nr_pages = min_t(unsigned long, nr_pages,
+				 2 * (PAGE_SIZE/sizeof(struct page)));
+
 	__SetPageHead(page);
 
 	for (i = 1; i < nr_pages; i++) {
@@ -6657,7 +6669,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
 			continue;
 
 		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
-				     pfns_per_compound);
+				     altmap, pfns_per_compound);
 	}
 
 	pr_info("%s initialised %lu pages in %ums\n", __func__,
-- 
2.17.1