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

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi Will,

> 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?

Because original code call the panic() when it fails to create
mapping of kpti_ng_temp_pgd and I think allocation is also part
of creating kpti_ng_temp_pgd too.
So, I added this panic() when allocation is failed for kpti_ng_temp_pgd.

--
Sincerely,
Yeoreum Yun