[RFC 4/4] mm/memblock: Skip initialization of struct pages freed later by HVO

[Usama Arif <usama.arif@bytedance.com>]

If the region is for hugepages and if HVO is enabled, then those
struct pages which will be freed later don't need to be initialized.
This can save significant time when a large number of hugepages are
allocated at boot time. As memmap_init_reserved_pages is only called at
boot time, we don't need to worry about memory hotplug.

Hugepage regions are kept separate from non hugepage regions in
memblock_merge_regions so that initialization for unused struct pages
can be skipped for the entire region.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
---
 mm/hugetlb_vmemmap.c |  2 +-
 mm/hugetlb_vmemmap.h |  3 +++
 mm/memblock.c        | 27 ++++++++++++++++++++++-----
 3 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index bdf750a4786b..b5b7834e0f42 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -443,7 +443,7 @@ static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
 DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);
 
-static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
+bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
 core_param(hugetlb_free_vmemmap, vmemmap_optimize_enabled, bool, 0);
 
 /**
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 3525c514c061..8b9a1563f7b9 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -58,4 +58,7 @@ static inline bool hugetlb_vmemmap_optimizable(const struct hstate *h)
 	return hugetlb_vmemmap_optimizable_size(h) != 0;
 }
 bool vmemmap_should_optimize(const struct hstate *h, const struct page *head);
+
+extern bool vmemmap_optimize_enabled;
+
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */
diff --git a/mm/memblock.c b/mm/memblock.c
index e92d437bcb51..62072a0226de 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -21,6 +21,7 @@
 #include <linux/io.h>
 
 #include "internal.h"
+#include "hugetlb_vmemmap.h"
 
 #define INIT_MEMBLOCK_REGIONS			128
 #define INIT_PHYSMEM_REGIONS			4
@@ -519,7 +520,8 @@ static void __init_memblock memblock_merge_regions(struct memblock_type *type,
 		if (this->base + this->size != next->base ||
 		    memblock_get_region_node(this) !=
 		    memblock_get_region_node(next) ||
-		    this->flags != next->flags) {
+		    this->flags != next->flags ||
+		    this->hugepage_size != next->hugepage_size) {
 			BUG_ON(this->base + this->size > next->base);
 			i++;
 			continue;
@@ -2125,10 +2127,25 @@ static void __init memmap_init_reserved_pages(void)
 	/* initialize struct pages for the reserved regions */
 	for_each_reserved_mem_region(region) {
 		nid = memblock_get_region_node(region);
-		start = region->base;
-		end = start + region->size;
-
-		reserve_bootmem_region(start, end, nid);
+		/*
+		 * If the region is for hugepages and if HVO is enabled, then those
+		 * struct pages which will be freed later don't need to be initialized.
+		 * This can save significant time when a large number of hugepages are
+		 * allocated at boot time. As this is at boot time, we don't need to
+		 * worry about memory hotplug.
+		 */
+		if (region->hugepage_size && vmemmap_optimize_enabled) {
+			for (start = region->base;
+			    start < region->base + region->size;
+			    start += region->hugepage_size) {
+				end = start + HUGETLB_VMEMMAP_RESERVE_SIZE * sizeof(struct page);
+				reserve_bootmem_region(start, end, nid);
+			}
+		} else {
+			start = region->base;
+			end = start + region->size;
+			reserve_bootmem_region(start, end, nid);
+		}
 	}
 }
 
-- 
2.25.1