Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while stop_machine()

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

Hi Brendan,

> On Sat, 13 Dec 2025 at 01:18, Yeoreum Yun <yeoreum.yun@arm.com> wrote:
> >
> > linear_map_split_to_ptes() and __kpti_install_ng_mappings()
> > are called as callback of stop_machine().
> > That means these functions context are preemption disabled.
> >
> > Unfortunately, under PREEMPT_RT, the pagetable_alloc() or
> > __get_free_pages() couldn't be called in this context
> > since spin lock that becomes sleepable on RT,
> > potentially causing a sleep during page allocation.
> >
> > To address this, pagetable_alloc_nolock().
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > ---
> >  arch/arm64/mm/mmu.c | 23 ++++++++++++++++++-----
> >  1 file changed, 18 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 2ba01dc8ef82..0e98606d8c4c 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -475,10 +475,15 @@ static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> >  static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
> >                                        enum pgtable_type pgtable_type)
> >  {
> > -       /* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
> > -       struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
> > +       struct ptdesc *ptdesc;
> >         phys_addr_t pa;
> >
> > +       /* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
> > +       if (gfpflags_allow_spinning(gfp))
> > +               ptdesc  = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
> > +       else
> > +               ptdesc  = pagetable_alloc_nolock(gfp & ~__GFP_ZERO, 0);
> > +
> >         if (!ptdesc)
> >                 return INVALID_PHYS_ADDR;
> >
> > @@ -869,6 +874,7 @@ static int __init linear_map_split_to_ptes(void *__unused)
> >                 unsigned long kstart = (unsigned long)lm_alias(_stext);
> >                 unsigned long kend = (unsigned long)lm_alias(__init_begin);
> >                 int ret;
> > +               gfp_t gfp = IS_ENABLED(CONFIG_PREEMPT_RT) ? __GFP_HIGH : GFP_ATOMIC;
> >
> >                 /*
> >                  * Wait for all secondary CPUs to be put into the waiting area.
> > @@ -881,9 +887,9 @@ static int __init linear_map_split_to_ptes(void *__unused)
> >                  * PTE. The kernel alias remains static throughout runtime so
> >                  * can continue to be safely mapped with large mappings.
> >                  */
> > -               ret = range_split_to_ptes(lstart, kstart, GFP_ATOMIC);
> > +               ret = range_split_to_ptes(lstart, kstart, gfp);
> >                 if (!ret)
> > -                       ret = range_split_to_ptes(kend, lend, GFP_ATOMIC);
> > +                       ret = range_split_to_ptes(kend, lend, gfp);
> >                 if (ret)
> >                         panic("Failed to split linear map\n");
> >                 flush_tlb_kernel_range(lstart, lend);
> > @@ -1207,7 +1213,14 @@ static int __init __kpti_install_ng_mappings(void *__unused)
> >         remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
> >
> >         if (!cpu) {
> > -               alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
> > +               if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +                       alloc = (u64) pagetable_alloc_nolock(__GFP_HIGH | __GFP_ZERO, order);
> > +               else
> > +                       alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
> > +
> > +               if (!alloc)
> > +                       panic("Failed to alloc kpti_ng_pgd\n");
> > +
>
> I don't have the context on what this code is doing so take this with
> a grain of salt, but...
>
> The point of the _nolock alloc is to give the allocator an excuse to
> fail. Panicking on that failure doesn't seem like a great idea to me?

I thought first whether it changes to "static" memory area to handle
this in PREEMPT_RT.
But since this function is called while smp_cpus_done().
So, I think it's fine since there wouldn't be a contention for
memory allocation in this phase.

Thanks.

--
Sincerely,
Yeoreum Yun