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

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Message-ID: <aT/drjN1BkvyAGoi@e129823.arm.com>
Date: Mon, 15 Dec 2025 10:06:38 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: akpm@...ux-foundation.org, david@...nel.org, lorenzo.stoakes@...cle.com,
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Subject: Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while
 stop_machine()

> On Mon Dec 15, 2025 at 9:34 AM UTC, Yeoreum Yun wrote:
> > Hi Brendan,
> >> On Sun Dec 14, 2025 at 9:13 AM UTC, Yeoreum Yun wrote:
> >> >> I don't have the context on what this code is doing so take this with
> >> >> a grain of salt, but...
> >> >>
> >> >> The point of the _nolock alloc is to give the allocator an excuse to
> >> >> fail. Panicking on that failure doesn't seem like a great idea to me?
> >> >
> >> > I thought first whether it changes to "static" memory area to handle
> >> > this in PREEMPT_RT.
> >> > But since this function is called while smp_cpus_done().
> >> > So, I think it's fine since there wouldn't be a contention for
> >> > memory allocation in this phase.
> >>
> >> Then shouldn't it use _nolock unconditionally?
> >
> > As you pointed out, I think it should be fine even in the !PREEMPT_RT case.
> > However, in case I missed something or if my understanding is incorrect,
> > I applied it only to the PREEMPT_RT case for now.
>
> Hmm, I don't think "this code might be broken so let's cage it behind a
> conditional" is a good strategy.
>
> 1. It bloats the codebase.
>
> 2. It's confusing to readers, now you have to try an understand why this
>    conditional is here, which is a doomed effort. This could be
>    mitigated with comments but, see point 1.
>
> 3. It expands the testing matrix. So now we have code that we aren't
>    really sure is correct, AND it gets less test coverage.
>
> 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.

--
Sincerely,
Yeoreum Yun