linux-kernel - Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while stop

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Message-ID: <DEZLRT59S25H.2YWTZ2G0TN3HV@google.com>
Date: Tue, 16 Dec 2025 11:26:29 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Yeoreum Yun <yeoreum.yun@....com>, Brendan Jackman <jackmanb@...gle.com>
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Subject: Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while stop_machine()

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.