Currently, hugetlb_cma reservation only supports absolute sizes (e.g.,
hugetlb_cma=2G or hugetlb_cma=0:1G,1:1G). This can be restrictive in
heterogeneous environments or when deploying common kernel command lines
across machines with different memory capacities.

Add support for percentage-based hugetlb_cma reservation (e.g.,
hugetlb_cma=20% or hugetlb_cma=0:20%,1:10%).

The percentage is calculated against the total memory (for global
settings) or against the node-specific memory (for node-specific
settings) using memblock APIs during early boot.

Signed-off-by: Sourav Panda <souravpanda@google.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
---
Link: https://lore.kernel.org/linux-mm/20260806085651.4068433-1-souravpanda@google.com/ [v4]
Link: https://lore.kernel.org/linux-mm/20260729163148.3271755-1-souravpanda@google.com/ [v3]
Link: https://lore.kernel.org/linux-mm/20260628190155.3655895-1-souravpanda@google.com/ [v2]
Link: https://lore.kernel.org/linux-mm/20260625215900.2151690-1-souravpanda@google.com/ [v1]

v5:
- Document in kernel-parameters.txt that percentage-derived sizes are rounded down to a multiple of the gigantic hugepage size and may become zero (Usama Arif).
- Add Acked-by: Usama Arif <usama.arif@linux.dev>.

v4:
- Clear global hugetlb_cma_percent and hugetlb_cma_size when node-specific parameters are parsed so the last command line configuration wins (Usama Arif).
- Warn when ALIGN_DOWN rounds down a non-zero percentage-derived reservation size to 0 MiB (Usama Arif).

v3:
- Enhance pr_info logs to explicitly show percentage-derived sizes per Andrew Morton's suggestions.
- Explicitly align percentage-derived sizes down to the gigantic page size to prevent allocation failures (Usama Arif).

 .../admin-guide/kernel-parameters.txt         |  10 +-
 mm/hugetlb_cma.c                              | 141 +++++++++++++++++-
 2 files changed, 141 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5a05b48d1684..160c414e0f30 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2064,8 +2064,14 @@ Kernel parameters
 	hugetlb_cma=	[HW,CMA,EARLY] The size of a CMA area used for allocation
 			of gigantic hugepages. Or using node format, the size
 			of a CMA area per node can be specified.
-			Format: nn[KMGTPE] or (node format)
-				<node>:nn[KMGTPE][,<node>:nn[KMGTPE]]
+			The size can be an absolute value (e.g., 2G) or a
+			percentage of the total memory or node memory (e.g., 20%).
+			Percentage-derived sizes are rounded down to a multiple of
+			the architecture's gigantic hugepage size and may become
+			zero.
+			Format: nn[KMGTPE] or nn% or (node format)
+				<node>:nn[KMGTPE][,<node>:nn[KMGTPE]] or
+				<node>:nn%[,<node>:nn%]
 
 			Reserve a CMA area of given size and allocate gigantic
 			hugepages using the CMA allocator. If enabled, the
diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
index 7693ccefd0c6..2ca014a0a3b8 100644
--- a/mm/hugetlb_cma.c
+++ b/mm/hugetlb_cma.c
@@ -9,6 +9,9 @@
 #include <asm/setup.h>
 
 #include <linux/hugetlb.h>
+#include <linux/memblock.h>
+#include <linux/math.h>
+#include <linux/math64.h>
 #include "internal.h"
 #include "hugetlb_cma.h"
 
@@ -18,6 +21,28 @@ static unsigned long hugetlb_cma_size_in_node[MAX_NUMNODES] __initdata;
 static bool hugetlb_cma_only __ro_after_init;
 static unsigned long hugetlb_cma_size __ro_after_init;
 
+static unsigned int hugetlb_cma_percent __initdata;
+static unsigned int hugetlb_cma_percent_in_node[MAX_NUMNODES] __initdata;
+
+#ifdef CONFIG_NUMA
+static phys_addr_t __init memblock_node_memory_size(int nid)
+{
+	struct memblock_region *reg;
+	phys_addr_t size = 0;
+
+	for_each_mem_region(reg) {
+		if (reg->nid == nid)
+			size += reg->size;
+	}
+	return size;
+}
+#else
+static phys_addr_t __init memblock_node_memory_size(int nid)
+{
+	return memblock_phys_mem_size();
+}
+#endif
+
 void hugetlb_cma_free_frozen_folio(struct folio *folio)
 {
 	WARN_ON_ONCE(!cma_release_frozen(hugetlb_cma[folio_nid(folio)],
@@ -100,14 +125,31 @@ static int __init cmdline_parse_hugetlb_cma(char *p)
 			break;
 
 		if (s[count] == ':') {
+			char *next;
+
 			if (tmp >= MAX_NUMNODES)
 				break;
 			nid = array_index_nospec(tmp, MAX_NUMNODES);
 
+			hugetlb_cma_size = 0;
+			hugetlb_cma_percent = 0;
+
 			s += count + 1;
-			tmp = memparse(s, &s);
-			hugetlb_cma_size_in_node[nid] = tmp;
-			hugetlb_cma_size += tmp;
+			tmp = memparse(s, &next);
+			if (*next == '%') {
+				if (tmp > 100) {
+					pr_warn("hugetlb_cma: invalid percentage %lu for node %d\n",
+						tmp, nid);
+					break;
+				}
+				hugetlb_cma_percent_in_node[nid] = tmp;
+				hugetlb_cma_size_in_node[nid] = 0;
+				s = next + 1;
+			} else {
+				hugetlb_cma_size_in_node[nid] = tmp;
+				hugetlb_cma_percent_in_node[nid] = 0;
+				s = next;
+			}
 
 			/*
 			 * Skip the separator if have one, otherwise
@@ -118,7 +160,28 @@ static int __init cmdline_parse_hugetlb_cma(char *p)
 			else
 				break;
 		} else {
-			hugetlb_cma_size = memparse(p, &p);
+			char *next;
+
+			tmp = memparse(p, &next);
+			if (*next == '%') {
+				if (tmp > 100) {
+					pr_warn("hugetlb_cma: invalid percentage %lu\n", tmp);
+				} else {
+					hugetlb_cma_percent = tmp;
+					hugetlb_cma_size = 0;
+					for (nid = 0; nid < MAX_NUMNODES; nid++) {
+						hugetlb_cma_size_in_node[nid] = 0;
+						hugetlb_cma_percent_in_node[nid] = 0;
+					}
+				}
+			} else {
+				hugetlb_cma_size = tmp;
+				hugetlb_cma_percent = 0;
+				for (nid = 0; nid < MAX_NUMNODES; nid++) {
+					hugetlb_cma_size_in_node[nid] = 0;
+					hugetlb_cma_percent_in_node[nid] = 0;
+				}
+			}
 			break;
 		}
 	}
@@ -144,8 +207,36 @@ void __init hugetlb_cma_reserve(void)
 {
 	unsigned long size, reserved, per_node, order;
 	bool node_specific_cma_alloc = false;
+	bool has_node_specific_param = false;
 	int nid;
 
+	for (nid = 0; nid < MAX_NUMNODES; nid++) {
+		if (hugetlb_cma_size_in_node[nid] || hugetlb_cma_percent_in_node[nid]) {
+			has_node_specific_param = true;
+			break;
+		}
+	}
+
+	if (has_node_specific_param) {
+		hugetlb_cma_size = 0;
+		for (nid = 0; nid < MAX_NUMNODES; nid++) {
+			if (hugetlb_cma_percent_in_node[nid]) {
+				phys_addr_t node_gfp_mem = memblock_node_memory_size(nid);
+				u64 s;
+
+				s = mul_u64_u32_div((u64)node_gfp_mem,
+						    hugetlb_cma_percent_in_node[nid],
+						    100);
+
+				hugetlb_cma_size_in_node[nid] = s;
+			}
+			hugetlb_cma_size += hugetlb_cma_size_in_node[nid];
+		}
+	} else if (hugetlb_cma_percent) {
+		hugetlb_cma_size = mul_u64_u32_div((u64)memblock_phys_mem_size(),
+						   hugetlb_cma_percent, 100);
+	}
+
 	if (!hugetlb_cma_size)
 		return;
 
@@ -163,6 +254,32 @@ void __init hugetlb_cma_reserve(void)
 	 */
 	VM_WARN_ON(order <= MAX_PAGE_ORDER);
 
+	if (hugetlb_cma_percent) {
+		unsigned long orig_size = hugetlb_cma_size;
+
+		hugetlb_cma_size = ALIGN_DOWN(hugetlb_cma_size, PAGE_SIZE << order);
+		if (orig_size && !hugetlb_cma_size)
+			pr_warn("hugetlb_cma: reservation size rounded down to 0 from %lu MiB (%u%%)\n",
+				orig_size / SZ_1M, hugetlb_cma_percent);
+	} else if (has_node_specific_param) {
+		hugetlb_cma_size = 0;
+		for (nid = 0; nid < MAX_NUMNODES; nid++) {
+			if (hugetlb_cma_percent_in_node[nid]) {
+				unsigned long orig_size = hugetlb_cma_size_in_node[nid];
+
+				hugetlb_cma_size_in_node[nid] =
+					ALIGN_DOWN(hugetlb_cma_size_in_node[nid],
+						   PAGE_SIZE << order);
+				if (orig_size && !hugetlb_cma_size_in_node[nid])
+					pr_warn("hugetlb_cma: reservation size rounded down to 0 from %lu MiB (%u%%) on node %d\n",
+						orig_size / SZ_1M,
+						hugetlb_cma_percent_in_node[nid],
+						nid);
+			}
+			hugetlb_cma_size += hugetlb_cma_size_in_node[nid];
+		}
+	}
+
 	hugetlb_bootmem_set_nodes();
 
 	for (nid = 0; nid < MAX_NUMNODES; nid++) {
@@ -205,8 +322,12 @@ void __init hugetlb_cma_reserve(void)
 		per_node = DIV_ROUND_UP(hugetlb_cma_size,
 					nodes_weight(hugetlb_bootmem_nodes));
 		per_node = round_up(per_node, PAGE_SIZE << order);
-		pr_info("hugetlb_cma: reserve %lu MiB, up to %lu MiB per node\n",
-			hugetlb_cma_size / SZ_1M, per_node / SZ_1M);
+		if (hugetlb_cma_percent)
+			pr_info("hugetlb_cma: reserve %lu MiB (%u%%), up to %lu MiB per node\n",
+				hugetlb_cma_size / SZ_1M, hugetlb_cma_percent, per_node / SZ_1M);
+		else
+			pr_info("hugetlb_cma: reserve %lu MiB, up to %lu MiB per node\n",
+				hugetlb_cma_size / SZ_1M, per_node / SZ_1M);
 	}
 
 	reserved = 0;
@@ -241,8 +362,12 @@ void __init hugetlb_cma_reserve(void)
 		}
 
 		reserved += size;
-		pr_info("hugetlb_cma: reserved %lu MiB on node %d\n",
-			size / SZ_1M, nid);
+		if (hugetlb_cma_percent_in_node[nid])
+			pr_info("hugetlb_cma: reserved %lu MiB (%u%%) on node %d\n",
+				size / SZ_1M, hugetlb_cma_percent_in_node[nid], nid);
+		else
+			pr_info("hugetlb_cma: reserved %lu MiB on node %d\n",
+				size / SZ_1M, nid);
 
 		if (reserved >= hugetlb_cma_size)
 			break;
-- 
2.55.0.679.g6767b8d81c-goog