[26/27] mm/hugetlb: add hugetlb_cma_only cmdline option

Commit Message

Frank van der Linden Jan. 27, 2025, 11:22 p.m. UTC

Add an option to force hugetlb gigantic pages to be allocated using
CMA only (if hugetlb_cma is enabled). This avoids a fallback to
allocation from the rest of system memory if the CMA allocation
fails. This makes the size of hugetlb_cma a hard upper boundary
for gigantic hugetlb page allocations.

This is useful because, with a large CMA area, the kernel's
unmovable allocations will have less room to work with and it
is undesirable for new hugetlb gigantic page allocations
to be done from that remaining area. It will eat in to the space
available for unmovable allocations, leading to unwanted system
behavior (OOMs because the kernel fails to do unmovable allocations).

So, with this enabled, an administrator can force a hard upper
bound for runtime gigantic page allocations, and have more
predictable system behavior.

Signed-off-by: Frank van der Linden <[e-mail]>
---
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
 mm/hugetlb.c                                    | 11 +++++++++++
 2 files changed, 18 insertions(+)

Patch

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d0f6c055dfcc..6a164466ec33 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1892,6 +1892,13 @@ 
 			hugepages using the CMA allocator. If enabled, the
 			boot-time allocation of gigantic hugepages is skipped.
 
+	hugetlb_cma_only=
+			[HW,CMA,EARLY] When allocating new HugeTLB pages, only
+			try to allocate from the CMA areas.
+
+			This option does nothing if hugetlb_cma= is not also
+			specified.
+
 	hugetlb_free_vmemmap=
 			[KNL] Requires CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 			enabled.
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 28653214f23d..32ebde9039e2 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -60,6 +60,7 @@  struct hstate hstates[HUGE_MAX_HSTATE];
 static struct cma *hugetlb_cma[MAX_NUMNODES];
 static unsigned long hugetlb_cma_size_in_node[MAX_NUMNODES] __initdata;
 #endif
+static bool hugetlb_cma_only;
 static unsigned long hugetlb_cma_size __initdata;
 
 __initdata struct list_head huge_boot_pages[MAX_NUMNODES];
@@ -1511,6 +1512,9 @@  static struct folio *alloc_gigantic_folio(struct hstate *h, gfp_t gfp_mask,
 	}
 #endif
 	if (!folio) {
+		if (hugetlb_cma_size && hugetlb_cma_only)
+			return NULL;
+
 		folio = folio_alloc_gigantic(order, gfp_mask, nid, nodemask);
 		if (!folio)
 			return NULL;
@@ -7844,6 +7848,13 @@  static int __init cmdline_parse_hugetlb_cma(char *p)
 
 early_param("hugetlb_cma", cmdline_parse_hugetlb_cma);
 
+static int __init cmdline_parse_hugetlb_cma_only(char *p)
+{
+	return kstrtobool(p, &hugetlb_cma_only);
+}
+
+early_param("hugetlb_cma_only", cmdline_parse_hugetlb_cma_only);
+
 void __init hugetlb_cma_reserve(int order)
 {
 	unsigned long size, reserved, per_node;