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

[Ryan Roberts <ryan.roberts@arm.com>]

On 18/12/2025 07:51, Vlastimil Babka wrote:
> 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?

I mean it's fine to use "GFP_KERNEL | __GFP_ZERO"; this context can sleep so we
don't want to dip into the reserves - we can sleep and reclaim if needed.

> 
>> 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)
>>
>