[PATCH v5 3/3] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI

Yeoreum Yun
2026-01-05T20:23:43Z

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@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
---
arch/arm64/mm/mmu.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 120874a2d35b..6ea5b80ab54f 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 +1411,16 @@ 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 +1433,14 @@ void __init kpti_install_ng_mappings(void)
if (arm64_use_ng_mappings)
return;

+ alloc = __get_free_pages(GFP_KERNEL | __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}