Hi Ryan,

On 18/12/2025 19:47, Yeoreum Yun wrote:

quoted

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 <redacted>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.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 96a9fa505e71..9ad9612728e6 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -1369,7 +1369,7 @@ 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;

@@ -1377,10 +1377,9 @@ 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;

@@ -1391,8 +1390,6 @@ 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);

@@ -1423,16 +1420,17 @@ 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;
+

nit: Restore the blank line between the variable definitioins and the logic.

Okay. I'll restore that line.

Thanks!

--
Sincerely,
Yeoreum Yun