[PATCH] mm/hugetlb_cma: restrict hugetlb_cma parameter to gigantic-page alignment

[PATCH] mm/hugetlb_cma: restrict hugetlb_cma parameter to gigantic-page alignment

[Sang-Heon Jeon]

Existing hugetlb_cma parameter handling logic rejects sizes smaller than
one gigantic page, but rounds up larger sizes that are not a multiple
of it. The two behaviors are inconsistent and neither is documented.

To remove existing inconsistent and undefined behavior, restrict
hugetlb_cma parameter to only accept multiples of the gigantic page size.

After this restriction, the redundant round_up() in the allocation loop
can be removed.

The new restriction is also documented in kernel-parameters.txt.

Also, including other minor changes for readability improvement with no
functional change.

Suggested-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
QEMU based test result

1) hugetlb_cma=1300M
  a) AS-IS : total 2G reserved
[    0.000000] hugetlb_cma: reserve 1300 MiB, up to 1024 MiB per node
[    0.000000] cma: Reserved 1024 MiB in 1 range
[    0.000000] hugetlb_cma: reserved 1024 MiB on node 0
[    0.000000] cma: Reserved 1024 MiB in 1 range
[    0.000000] hugetlb_cma: reserved 1024 MiB on node 1

  b) TO-BE : rejected
[    0.000000] hugetlb_cma: cma area must be a multiple of 1024 MiB

2) hugetlb_cma=0:1300M,1:1G
  a) AS-IS : 2G reserved on node 0, 1G reserved on node 1
[    0.000000] cma: Reserved 2048 MiB in 1 range
[    0.000000] hugetlb_cma: reserved 2048 MiB on node 0
[    0.000000] cma: Reserved 1024 MiB in 1 range
[    0.000000] hugetlb_cma: reserved 1024 MiB on node 1

  b) TO-BE : 1G reserved on node 1
[    0.000000] hugetlb_cma: cma area of node 0 must be a multiple of 1024 MiB
[    0.000000] cma: Reserved 1024 MiB in 1 range
[    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
---
Hello,

This patch implements a new restriction to hugetlb_cma parameter that
was suggested during the review of the previous patch [1].
Thanks to Muchun for the guidance.

[1] https://lore.kernel.org/all/BFB1F124-B599-4832-A9DA-F4931999BAF9@linux.dev/

Best Regards,
Sang-Heon Jeon
---
 .../admin-guide/kernel-parameters.txt         |  4 +++
 mm/hugetlb_cma.c                              | 35 +++++++++----------
 2 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 4d0f545fb3ec..23be2f64439c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2100,6 +2100,10 @@ Kernel parameters
 			Format: nn[KMGTPE] or (node format)
 				<node>:nn[KMGTPE][,<node>:nn[KMGTPE]]
 
+			The size must be a multiple of the gigantic page size.
+			When using node format, this applies to each per-node size.
+			Missaligned values are dropped with a warning.
+
 			Reserve a CMA area of given size and allocate gigantic
 			hugepages using the CMA allocator. If enabled, the
 			boot-time allocation of gigantic hugepages is skipped.
diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
index 7693ccefd0c6..39344d6c78d8 100644
--- a/mm/hugetlb_cma.c
+++ b/mm/hugetlb_cma.c
@@ -142,7 +142,7 @@ unsigned int __weak arch_hugetlb_cma_order(void)
 
 void __init hugetlb_cma_reserve(void)
 {
-	unsigned long size, reserved, per_node, order;
+	unsigned long size, reserved, per_node, order, gigantic_page_size;
 	bool node_specific_cma_alloc = false;
 	int nid;
 
@@ -162,37 +162,36 @@ void __init hugetlb_cma_reserve(void)
 	 * breaking this assumption.
 	 */
 	VM_WARN_ON(order <= MAX_PAGE_ORDER);
+	gigantic_page_size = PAGE_SIZE << order;
 
 	hugetlb_bootmem_set_nodes();
 
 	for (nid = 0; nid < MAX_NUMNODES; nid++) {
-		if (hugetlb_cma_size_in_node[nid] == 0)
+		size = hugetlb_cma_size_in_node[nid];
+		if (size == 0)
 			continue;
 
 		if (!node_isset(nid, hugetlb_bootmem_nodes)) {
 			pr_warn("hugetlb_cma: invalid node %d specified\n", nid);
-			hugetlb_cma_size -= hugetlb_cma_size_in_node[nid];
-			hugetlb_cma_size_in_node[nid] = 0;
-			continue;
-		}
-
-		if (hugetlb_cma_size_in_node[nid] < (PAGE_SIZE << order)) {
-			pr_warn("hugetlb_cma: cma area of node %d should be at least %lu MiB\n",
-				nid, (PAGE_SIZE << order) / SZ_1M);
-			hugetlb_cma_size -= hugetlb_cma_size_in_node[nid];
-			hugetlb_cma_size_in_node[nid] = 0;
+		} else if (!IS_ALIGNED(size, gigantic_page_size)) {
+			pr_warn("hugetlb_cma: cma area of node %d must be a multiple of %lu MiB\n",
+				nid, gigantic_page_size / SZ_1M);
 		} else {
 			node_specific_cma_alloc = true;
+			continue;
 		}
+
+		hugetlb_cma_size -= size;
+		hugetlb_cma_size_in_node[nid] = 0;
 	}
 
 	/* Validate the CMA size again in case some invalid nodes specified. */
 	if (!hugetlb_cma_size)
 		return;
 
-	if (hugetlb_cma_size < (PAGE_SIZE << order)) {
-		pr_warn("hugetlb_cma: cma area should be at least %lu MiB\n",
-			(PAGE_SIZE << order) / SZ_1M);
+	if (!IS_ALIGNED(hugetlb_cma_size, gigantic_page_size)) {
+		pr_warn("hugetlb_cma: cma area must be a multiple of %lu MiB\n",
+			gigantic_page_size / SZ_1M);
 		hugetlb_cma_size = 0;
 		return;
 	}
@@ -204,7 +203,7 @@ 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);
+		per_node = round_up(per_node, gigantic_page_size);
 		pr_info("hugetlb_cma: reserve %lu MiB, up to %lu MiB per node\n",
 			hugetlb_cma_size / SZ_1M, per_node / SZ_1M);
 	}
@@ -223,15 +222,13 @@ void __init hugetlb_cma_reserve(void)
 			size = min(per_node, hugetlb_cma_size - reserved);
 		}
 
-		size = round_up(size, PAGE_SIZE << order);
-
 		snprintf(name, sizeof(name), "hugetlb%d", nid);
 		/*
 		 * Note that 'order per bit' is based on smallest size that
 		 * may be returned to CMA allocator in the case of
 		 * huge page demotion.
 		 */
-		res = cma_declare_contiguous_multi(size, PAGE_SIZE << order,
+		res = cma_declare_contiguous_multi(size, gigantic_page_size,
 					HUGETLB_PAGE_ORDER, name,
 					&hugetlb_cma[nid], nid);
 		if (res) {
-- 
2.43.0

Re: [PATCH] mm/hugetlb_cma: restrict hugetlb_cma parameter to gigantic-page alignment

[Muchun Song]

> On May 3, 2026, at 16:42, Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> 
> Existing hugetlb_cma parameter handling logic rejects sizes smaller than
> one gigantic page, but rounds up larger sizes that are not a multiple
> of it. The two behaviors are inconsistent and neither is documented.
> 
> To remove existing inconsistent and undefined behavior, restrict
> hugetlb_cma parameter to only accept multiples of the gigantic page size.
> 
> After this restriction, the redundant round_up() in the allocation loop
> can be removed.
> 
> The new restriction is also documented in kernel-parameters.txt.
> 
> Also, including other minor changes for readability improvement with no
> functional change.
> 
> Suggested-by: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
> QEMU based test result
> 
> 1) hugetlb_cma=1300M
>  a) AS-IS : total 2G reserved
> [    0.000000] hugetlb_cma: reserve 1300 MiB, up to 1024 MiB per node
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 0
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> 
>  b) TO-BE : rejected
> [    0.000000] hugetlb_cma: cma area must be a multiple of 1024 MiB
> 
> 2) hugetlb_cma=0:1300M,1:1G
>  a) AS-IS : 2G reserved on node 0, 1G reserved on node 1
> [    0.000000] cma: Reserved 2048 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 2048 MiB on node 0
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> 
>  b) TO-BE : 1G reserved on node 1
> [    0.000000] hugetlb_cma: cma area of node 0 must be a multiple of 1024 MiB
> [    0.000000] cma: Reserved 1024 MiB in 1 range
> [    0.000000] hugetlb_cma: reserved 1024 MiB on node 1
> ---
> Hello,
> 
> This patch implements a new restriction to hugetlb_cma parameter that
> was suggested during the review of the previous patch [1].
> Thanks to Muchun for the guidance.
> 
> [1] https://lore.kernel.org/all/BFB1F124-B599-4832-A9DA-F4931999BAF9@linux.dev/
> 
> Best Regards,
> Sang-Heon Jeon
> ---
> .../admin-guide/kernel-parameters.txt         |  4 +++
> mm/hugetlb_cma.c                              | 35 +++++++++----------
> 2 files changed, 20 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4d0f545fb3ec..23be2f64439c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2100,6 +2100,10 @@ Kernel parameters
> 			Format: nn[KMGTPE] or (node format)
> 			<node>:nn[KMGTPE][,<node>:nn[KMGTPE]]
> 
> + 			The size must be a multiple of the gigantic page size.
> + 			When using node format, this applies to each per-node size.
> + 			Missaligned values are dropped with a warning.
                            ^
			 Misaligned

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.

Re: [PATCH] mm/hugetlb_cma: restrict hugetlb_cma parameter to gigantic-page alignment

[Oscar Salvador]

On Sun, May 03, 2026 at 05:42:25PM +0900, Sang-Heon Jeon wrote:
> Existing hugetlb_cma parameter handling logic rejects sizes smaller than
> one gigantic page, but rounds up larger sizes that are not a multiple
> of it. The two behaviors are inconsistent and neither is documented.
> 
> To remove existing inconsistent and undefined behavior, restrict
> hugetlb_cma parameter to only accept multiples of the gigantic page size.
> 
> After this restriction, the redundant round_up() in the allocation loop
> can be removed.
> 
> The new restriction is also documented in kernel-parameters.txt.
> 
> Also, including other minor changes for readability improvement with no
> functional change.
> 
> Suggested-by: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>

Acked-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs