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

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Message-ID: <731e3f48-e0a3-4c89-a13a-65ef37faa443@arm.com>
Date: Wed, 17 Dec 2025 19:03:02 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: 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,
 vbabka@...e.cz
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 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?

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)