[PATCH v5 3/3] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI
From: Yeoreum Yun <hidden>
Date: 2026-01-05 20:23:43
Also in:
linux-rt-devel, lkml
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
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 | 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}