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

Vlastimil Babka
2025-12-18T07:51:55Z

On 12/17/25 20:03, Ryan Roberts wrote:
> On 17/12/2025 18:20, 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>
>> ---
>> 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);
>
> I don't think this requires GFP_ATOMIC now?

Do you mean it's fine to use GFP_KERNEL now, or still not, and you mean that
GFP_NOWAIT is sufficient?

> With that removed:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>
>> + 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)
>