linux-kernel - [PATCH v2 2/2] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI

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Message-Id: <20251217182007.2345700-3-yeoreum.yun@arm.com>
Date: Wed, 17 Dec 2025 18:20:07 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: catalin.marinas@....com,
	will@...nel.org,
	ryan.roberts@....com,
	akpm@...ux-foundation.org,
	david@...nel.org,
	kevin.brodsky@....com,
	quic_zhenhuah@...cinc.com,
	dev.jain@....com,
	yang@...amperecomputing.com,
	chaitanyas.prakash@....com,
	bigeasy@...utronix.de,
	clrkwllms@...nel.org,
	rostedt@...dmis.org,
	lorenzo.stoakes@...cle.com,
	ardb@...nel.org,
	jackmanb@...gle.com,
	vbabka@...e.cz
Cc: linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev,
	Yeoreum Yun <yeoreum.yun@....com>
Subject: [PATCH v2 2/2] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI

The current __kpti_install_ng_mappings() allocates a temporary PGD
while installing the NG mapping for KPTI under stop_machine(),
using GFP_ATOMIC.

This is fine in the non-PREEMPT_RT case. However, it becomes a problem
under PREEMPT_RT because generic memory allocation/free APIs
(e.g., pgtable_alloc(), __get_free_pages(), etc.) cannot be invoked
in a non-preemptible context, except for the *_nolock() variants.
These generic allocators may sleep due to their use of spin_lock().

In other words, calling __get_free_pages(), even with GFP_ATOMIC,
is not allowed in __kpti_install_ng_mappings(), which is executed by
the stopper thread where preemption is disabled under PREEMPT_RT.

To address this, preallocate the page needed for the temporary PGD
before invoking __kpti_install_ng_mappings() via stop_machine().

Fixes: 47546a1912fc ("arm64: mm: install KPTI nG mappings with MMU enabled")
Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
---
 arch/arm64/mm/mmu.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e4e6c7e0a016..69d9651de0cd 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1360,7 +1360,7 @@ static phys_addr_t __init kpti_ng_pgd_alloc(enum pgtable_type type)
 	return kpti_ng_temp_alloc;
 }
 
-static int __init __kpti_install_ng_mappings(void *__unused)
+static int __init __kpti_install_ng_mappings(void *data)
 {
 	typedef void (kpti_remap_fn)(int, int, phys_addr_t, unsigned long);
 	extern kpti_remap_fn idmap_kpti_install_ng_mappings;
@@ -1368,10 +1368,9 @@ static int __init __kpti_install_ng_mappings(void *__unused)
 
 	int cpu = smp_processor_id();
 	int levels = CONFIG_PGTABLE_LEVELS;
-	int order = order_base_2(levels);
 	u64 kpti_ng_temp_pgd_pa = 0;
 	pgd_t *kpti_ng_temp_pgd;
-	u64 alloc = 0;
+	u64 alloc = *(u64 *)data;
 
 	if (levels == 5 && !pgtable_l5_enabled())
 		levels = 4;
@@ -1382,8 +1381,6 @@ static int __init __kpti_install_ng_mappings(void *__unused)
 
 	if (!cpu) {
 		int ret;
-
-		alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
 		kpti_ng_temp_pgd = (pgd_t *)(alloc + (levels - 1) * PAGE_SIZE);
 		kpti_ng_temp_alloc = kpti_ng_temp_pgd_pa = __pa(kpti_ng_temp_pgd);
 
@@ -1414,16 +1411,17 @@ static int __init __kpti_install_ng_mappings(void *__unused)
 	remap_fn(cpu, num_online_cpus(), kpti_ng_temp_pgd_pa, KPTI_NG_TEMP_VA);
 	cpu_uninstall_idmap();
 
-	if (!cpu) {
-		free_pages(alloc, order);
+	if (!cpu)
 		arm64_use_ng_mappings = true;
-	}
 
 	return 0;
 }
 
 void __init kpti_install_ng_mappings(void)
 {
+	int order = order_base_2(CONFIG_PGTABLE_LEVELS);
+	u64 alloc;
+
 	/* Check whether KPTI is going to be used */
 	if (!arm64_kernel_unmapped_at_el0())
 		return;
@@ -1436,8 +1434,14 @@ void __init kpti_install_ng_mappings(void)
 	if (arm64_use_ng_mappings)
 		return;
 
+	alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
+	if (!alloc)
+		panic("Failed to alloc page tables\n");
+
 	init_idmap_kpti_bbml2_flag();
-	stop_machine(__kpti_install_ng_mappings, NULL, cpu_online_mask);
+	stop_machine(__kpti_install_ng_mappings, &alloc, cpu_online_mask);
+
+	free_pages(alloc, order);
 }
 
 static pgprot_t __init kernel_exec_prot(void)
-- 
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}