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

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

> On Tue Dec 16, 2025 at 11:03 AM UTC, Yeoreum Yun wrote:
> > Hi Brendan,
> >
> >> On Mon Dec 15, 2025 at 10:06 AM UTC, Yeoreum Yun wrote:
> >> [snip]
> >> >> Overall I am feeling a bit uncomfortable about this use of _nolock, but
> >> >> I am also feeling pretty ignorant about PREEMPT_RT and also about this
> >> >> arm64 code, so I am hesitant to suggest alternatives, I hope someone
> >> >> else can offer some input here...
> >> >
> >> > I understand. However, as I mentioned earlier,
> >> > my main intention was to hear opinions specifically about memory contention.
> >> >
> >> > That said, if there is no memory contention,
> >> > I don’t think using the _nolock API is necessarily a bad approach.
> >>
> >>
> >> > In fact, I believe a bigger issue is that, under PREEMPT_RT,
> >> > code that uses the regular memory allocation APIs may give users the false impression
> >> > that those APIs are “safe to use,” even though they are not.
> >>
> >> Yeah, I share this concern. I would bet I have written code that's
> >> broken under PREEMPT_RT (luckily only in Google's kernel fork). The
> >> comment for GFP_ATOMIC says:
> >>
> >>  * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> >>  * watermark is applied to allow access to "atomic reserves".
> >>  * The current implementation doesn't support NMI and few other strict
> >>  * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.
> >>
> >> It kinda sounds like it's supposed to be OK to use GFP_ATOMIC in a
> >> normal preempt_disable() context. So do you know exactly why it's
> >> invalid to use it in this stop_machine() context here? Maybe we need to
> >> update this comment.
> >
> > In non-PREEMPT_RT configurations, this is fine to use.
> > However, in PREEMPT_RT, it should not be used because
> > spin_lock becomes a sleepable lock backed by an rt-mutex.
> >
> > From Documentation/locking/locktypes.rst:
> >
> >   The fact that PREEMPT_RT changes the lock category of spinlock_t and
> >   rwlock_t from spinning to sleeping.
> >
> > As you know, all locks related to memory allocation
> > (e.g., zone_lock, PCP locks, etc.) use spin_lock,
> > which becomes sleepable under PREEMPT_RT.
> >
> > The callback of stop_machine() is executed in a preemption-disabled context
> > (see cpu_stopper_thread()). In this context, if it fails to acquire a spinlock
> > during memory allocation,
> > the task would be able to go to sleep while preemption is disabled,
> > which is an obviously problematic situation.
>
> But this is what I mean, doesn't this sound like the GFP_ATOMIC comment
> I quoted is wrong (or at least, it implies things which are wrong)? The
> comment refers specifically to raw_spin_lock() and "strict
> non-preemptive contexts". Which sounds like it is being written with
> PREEMPT_RT in mind. But that doesn't really match what you've said.

No. I think the comment of GFP_ATOMIC is right.
It definitely said:
  The current implementation *doesn't support* NMI and few other strict
  *non-preemptive contexts (e.g. raw_spin_lock)*.

The reason It couldn't be support GFP_ATOMIC in raw_spin_lock() context
in PREEMPT_RT since critical section protected by raw_spin_lock()
is non-preemptive (preemption disabled).

This is the same reason "GFP_ATOMIC" cannot be used in the
stop_machine().

--
Sincerely,
Yeoreum Yun