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

Yeoreum Yun
2025-12-18T09:37:21Z

Hi,
> On Fri 12-12-25 16:18:32, Yeoreum Yun 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().
>
> As you cannot tolerate allocation failure and this is pretty much
> permanent allocation (AFAIU) why don't you use a static allocation?

Because of when bbl2_noabort is supported, that pages doesn't need to.
If static alloc, that would be a waste in the system where bbl2_noabort
is supported.

When I tested, these extra pages are more than 40 in my FVP.
So, it would be better dynamic allocation and I think since it's quite a
early time, it's probably not failed that's why former code runs as it
is.

--
Sincerely,
Yeoreum Yun