[PATCH v2 2/2] memblock: use binary search to locate candidate regions

[Tarun Sahu]

Use binary search (memblock_bsearch_start) in memblock_add_range() and
memblock_isolate_range() to locate candidate regions instead of linearly
scanning from index 0.

Under heavy memory fragmentation (such as KHO page preservation registering
hundreds of thousands of disjoint folios), scanning from index 0 on every
insertion and isolation results in O(N^2) complexity, causing boot-time
memory retrieval to take several minutes (~268s for 393k pages).

Using binary search reduces the worst-case complexity to O(N log N)
(and O(N) for sequential appends), cutting KHO memory retrieval time
from ~268s to ~50ms.

memblock_search() open codes the same binary search, so reimplement it on
top of the new helper.

Signed-off-by: Tarun Sahu <tarunsahu at google.com>
---
 mm/memblock.c | 53 +++++++++++++++++++++++++++++++++++----------------
 1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index 5aae75d34dec..00a8768de988 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -586,6 +586,33 @@ static void __init_memblock memblock_insert_region(struct memblock_type *type,
 	type->total_size += size;
 }
 
+/**
+ * memblock_bsearch_start - Find the first region index where rend > base
+ * @type: memblock type to search
+ * @base: base physical address of the candidate range
+ *
+ * Returns the first region index that could potentially overlap @base.
+ */
+static int __init_memblock memblock_bsearch_start(struct memblock_type *type,
+						  phys_addr_t base)
+{
+	int mid, low = 0;
+	int high = type->cnt;
+
+	if (type->cnt && base >= type->regions[type->cnt - 1].base +
+				 type->regions[type->cnt - 1].size)
+		return type->cnt;
+
+	while (low < high) {
+		mid = (low + high) / 2;
+		if (type->regions[mid].base + type->regions[mid].size <= base)
+			low = mid + 1;
+		else
+			high = mid;
+	}
+	return low;
+}
+
 /**
  * memblock_add_range - add new memblock region
  * @type: memblock type to add new region into
@@ -609,7 +636,7 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
 	bool insert = false;
 	phys_addr_t obase = base;
 	phys_addr_t end = base + memblock_cap_size(base, &size);
-	int idx, nr_new, start_rgn = -1, end_rgn;
+	int idx, start_idx, nr_new, start_rgn = -1, end_rgn;
 	struct memblock_region *rgn;
 
 	if (!size)
@@ -645,8 +672,9 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
 	 */
 	base = obase;
 	nr_new = 0;
+	start_idx = memblock_bsearch_start(type, base);
 
-	for (idx = 0; idx < type->cnt; idx++) {
+	for (idx = start_idx; idx < type->cnt; idx++) {
 		rgn = &type->regions[idx];
 		phys_addr_t rbase = rgn->base;
 		phys_addr_t rend = rbase + rgn->size;
@@ -810,7 +838,7 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
 					int *start_rgn, int *end_rgn)
 {
 	phys_addr_t end = base + memblock_cap_size(base, &size);
-	int idx;
+	int idx, start_idx;
 	struct memblock_region *rgn;
 
 	*start_rgn = *end_rgn = 0;
@@ -823,7 +851,9 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
 		if (memblock_double_array(type, base, size) < 0)
 			return -ENOMEM;
 
-	for (idx = 0; idx < type->cnt; idx++) {
+	start_idx = memblock_bsearch_start(type, base);
+
+	for (idx = start_idx; idx < type->cnt; idx++) {
 		rgn = &type->regions[idx];
 		phys_addr_t rbase = rgn->base;
 		phys_addr_t rend = rbase + rgn->size;
@@ -2064,19 +2094,10 @@ void __init memblock_mem_limit_remove_map(phys_addr_t limit)
 
 static int __init_memblock memblock_search(struct memblock_type *type, phys_addr_t addr)
 {
-	unsigned int left = 0, right = type->cnt;
+	int idx = memblock_bsearch_start(type, addr);
 
-	do {
-		unsigned int mid = (right + left) / 2;
-
-		if (addr < type->regions[mid].base)
-			right = mid;
-		else if (addr >= (type->regions[mid].base +
-				  type->regions[mid].size))
-			left = mid + 1;
-		else
-			return mid;
-	} while (left < right);
+	if (idx < type->cnt && addr >= type->regions[idx].base)
+		return idx;
 	return -1;
 }
 
-- 
2.55.0.1082.g2b9226bbc0-goog