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

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Message-ID: <586d24f9-a54d-4c14-bd25-fed18423d709@suse.cz>
Date: Thu, 18 Dec 2025 08:51:52 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Ryan Roberts <ryan.roberts@....com>, Yeoreum Yun <yeoreum.yun@....com>,
 catalin.marinas@....com, will@...nel.org, akpm@...ux-foundation.org,
 david@...nel.org, kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
 dev.jain@....com, yang@...amperecomputing.com, chaitanyas.prakash@....com,
 bigeasy@...utronix.de, clrkwllms@...nel.org, rostedt@...dmis.org,
 lorenzo.stoakes@...cle.com, ardb@...nel.org, jackmanb@...gle.com
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] arm64: mmu: avoid allocating pages while
 installing ng-mapping for KPTI

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@....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@....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)
>