[PATCH v5] mm/hugetlb: ignore hugepage kernel args if hugepages are unsupported

[Sourabh Jain]

Skip processing hugepage kernel arguments (hugepagesz, hugepages, and
default_hugepagesz) when hugepages are not supported by the
architecture.

Some architectures may need to disable hugepages based on conditions
discovered during kernel boot. The hugepages_supported() helper allows
architecture code to advertise whether hugepages are supported.

Currently, normal hugepage allocation is guarded by
hugepages_supported(), but gigantic hugepages are allocated regardless
of this check. This causes problems on powerpc for fadump (firmware-
assisted dump).

In the fadump (firmware-assisted dump) scenario, a production kernel
crash causes the system to boot into a special kernel whose sole
purpose is to collect the memory dump and reboot. Features such as
hugepages are not required in this environment and should be
disabled.

For example, fadump kernel booting with the kernel arguments
default_hugepagesz=1GB hugepagesz=1GB hugepages=200 prints the
following logs:

HugeTLB: allocating 200 of page size 1.00 GiB failed.  Only allocated 58 hugepages.
HugeTLB support is disabled!
HugeTLB: huge pages not supported, ignoring associated command-line parameters
hugetlbfs: disabling because there are no supported hugepage sizes

Even though the logs say that hugetlb support is disabled, gigantic
hugepages are still getting allocated, which causes the fadump kernel
to run out of memory during boot.

To fix this, the gigantic hugepage allocation should come under
hugepages_supported().

To bring gigantic hugepage allocation under hugepages_supported(), two
approaches were previously proposed:
[1] Check hugepages_supported() in the generic code before allocating
gigantic hugepages.
[2] Make arch_hugetlb_valid_size() return false for all hugetlb sizes.

Approach [2] has two minor issues:
1. It prints misleading logs about invalid hugepage sizes
2. The kernel still processes hugepage kernel arguments unnecessarily

To control gigantic hugepage allocation, it is proposed to skip
processing the hugepage kernel arguments (hugepagesz, hugepages, and
default_hugepagesz) when hugepages_support() returns false.

Link: https://lore.kernel.org/all/20250121150419.1342794-1-sourabhjain@linux.ibm.com/ [1]
Link: https://lore.kernel.org/all/20250128043358.163372-1-sourabhjain@linux.ibm.com/ [2]
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Christophe Leroy <christophe.leroy at csgroup.eu>
Cc: David Hildenbrand <david at kernel.org>
Cc: Heiko Carstens <hca at linux.ibm.com>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Madhavan Srinivasan <maddy at linux.ibm.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Muchun Song <muchun.song at linux.dev>
Cc: Oscar Salvador <osalvador at suse.de>
Cc: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Vasily Gorbik <gor at linux.ibm.com>
Cc: linux-mm at kvack.org
Cc: linuxppc-dev at lists.ozlabs.org
Cc: linux-s390 at vger.kernel.org
Cc: x86 at kernel.org
Cc: linux-arm64 at lists.infradead.org
Cc: linux-riscv at lists.infradead.org
Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>
---

Changelog:

v1:
https://lore.kernel.org/all/20250121150419.1342794-1-sourabhjain@linux.ibm.com/

v2:
https://lore.kernel.org/all/20250124103220.111303-1-sourabhjain@linux.ibm.com/
 - disable gigantic hugepage in arch code, arch_hugetlb_valid_size()

v3:
https://lore.kernel.org/all/20250125104928.88881-1-sourabhjain@linux.ibm.com/
 - Do not modify the initialization of the shift variable

v4:
https://lore.kernel.org/all/20250128043358.163372-1-sourabhjain@linux.ibm.com/
 - Update commit message to include how hugepages_supported() detects
   hugepages support when fadump is active
 - Add Reviewed-by tag
 - NO functional change

v5:
 - Significant change in approach: disable processing of hugepage kernel
   arguments if hugepages_supported() returns false
 - Drop a Reviewed-by tag

---
 mm/hugetlb.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 51273baec9e5..e0ab14020513 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4286,6 +4286,11 @@ static int __init hugepages_setup(char *s)
 	unsigned long tmp;
 	char *p = s;
 
+	if (!hugepages_supported()) {
+		pr_warn("HugeTLB: hugepages unsupported, ignoring hugepages=%s cmdline\n", s);
+		return 0;
+	}
+
 	if (!parsed_valid_hugepagesz) {
 		pr_warn("HugeTLB: hugepages=%s does not follow a valid hugepagesz, ignoring\n", s);
 		parsed_valid_hugepagesz = true;
@@ -4366,6 +4371,11 @@ static int __init hugepagesz_setup(char *s)
 	unsigned long size;
 	struct hstate *h;
 
+	if (!hugepages_supported()) {
+		pr_warn("HugeTLB: hugepages unsupported, ignoring hugepagesz=%s cmdline\n", s);
+		return 0;
+	}
+
 	parsed_valid_hugepagesz = false;
 	size = (unsigned long)memparse(s, NULL);
 
@@ -4414,6 +4424,12 @@ static int __init default_hugepagesz_setup(char *s)
 	unsigned long size;
 	int i;
 
+	if (!hugepages_supported()) {
+		pr_warn("HugeTLB: hugepages unsupported, ignoring default_hugepagesz=%s cmdline\n",
+			s);
+		return 0;
+	}
+
 	parsed_valid_hugepagesz = false;
 	if (parsed_default_hugepagesz) {
 		pr_err("HugeTLB: default_hugepagesz previously specified, ignoring %s\n", s);
-- 
2.51.1