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

Will Deacon
2026-01-19T17:31:49Z

On Mon, Jan 05, 2026 at 08:23:28PM +0000, Yeoreum Yun wrote:
> 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");

Why are you adding this panic?

Will