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

[Brendan Jackman <jackmanb@google.com>]

On Tue Dec 16, 2025 at 12:01 PM UTC, Yeoreum Yun wrote:
>> 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)*.

But this phrasing sounds like there are other non-preemptive contexts
that it _does_ support. I would definitely read this as implying that
plain old preempt_disable() is OK. I don't understand what those "few
other strict contexts" are, nor why the stop_machine() context is
included in them.