Re: [PATCH 0/2] introduce pagetable_alloc_nolock()

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

On 12/12/2025 16:18, Yeoreum Yun wrote:
> Some architectures invoke pagetable_alloc() or __get_free_pages()
> with preemption disabled.
> For example, in arm64, linear_map_split_to_ptes() calls pagetable_alloc()
> while spliting block entry to ptes and __kpti_install_ng_mappings()
> calls __get_free_pages() to create kpti pagetable.
> 
> Under PREEMPT_RT, calling pagetable_alloc() with
> preemption disabled is not allowed, because it may acquire
> a spin lock that becomes sleepable on RT, potentially
> causing a sleep during page allocation.
> 
> Since above two functions is called as callback of stop_machine()
> where its callback is called in preemption disabled,
> They could make a potential problem. (sleeping in preemption disabled).
> 
> To address this, introduce pagetable_alloc_nolock() API.

I don't really understand what the problem is that you're trying to fix. As I
see it, there are 2 call sites in arm64 arch code that are calling into the page
allocator from stop_machine() - one via via pagetable_alloc() and another via
__get_free_pages(). But both of those calls are passing in GFP_ATOMIC. It was my
understanding that the page allocator would ensure it never sleeps when
GFP_ATOMIC is passed in, (even for PREEMPT_RT)?

What is the actual symptom you are seeing?

If the page allocator is somehow ignoring the GFP_ATOMIC request for PREEMPT_RT,
then isn't that a bug in the page allocator? I'm not sure why you would change
the callsites? Can't you just change the page allocator based on GFP_ATOMIC?

Thanks,
Ryan

> 
> Yeoreum Yun (2):
>   mm: introduce pagetable_alloc_nolock()
>   arm64: mmu: use pagetable_alloc_nolock() while stop_machine()
> 
>  arch/arm64/mm/mmu.c  | 23 ++++++++++++++++++-----
>  include/linux/mm.h   | 18 ++++++++++++++++++
>  kernel/bpf/stream.c  |  2 +-
>  kernel/bpf/syscall.c |  2 +-
>  mm/page_alloc.c      | 10 +++-------
>  5 files changed, 41 insertions(+), 14 deletions(-)
> 
> --
> LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
>