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Subject: Re: ipv4/tcp.c:4234:1: error: the frame size of 1152 bytes is larger
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
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From:   Nathan Chancellor <nathan@kernel.org>
Message-ID: <92c20b62-c4a7-8e63-4a94-76bdf6d9481e@kernel.org>
Date:   Tue, 7 Sep 2021 16:35:26 -0700
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On 9/7/2021 4:14 PM, Linus Torvalds wrote:
> There are many more of these cases. I've seen Hyper-V allocate 'struct
> cpumask' on the stack, which is once again an absolute no-no that
> people have apparently just ignored the warning for. When you have
> NR_CPUS being the maximum of 8k, those bits add up, and a single
> cpumask is 1kB in size. Which is why you should never do that on
> stack, and instead use '
> 
>         cpumask_var_t mask;
>         alloc_cpumask_var(&mask,..)
> 
> which will do a much more reasonable job. But the reason I call out
> hyperv is that as far as I know, hyperv itself doesn't actually
> support 8192 CPU's. So all that apic noise with 'struct cpumask' that
> uses 1kB of data when NR_CPUS is set to 8192 is just wasted. Maybe I'm
> wrong. Adding hyperv people to the cc too.

I am only commenting on this because I was looking into an instance of 
this warning yesterday with Fedora's arm64 config, which has 
CONFIG_NR_CPUS=4096:

https://lore.kernel.org/r/YTZyMx91zV9kfDkQ@Ryzen-9-3900X.localdomain/

Won't your example only fix the issue with CONFIG_CPUMASK_OFFSTACK=y or 
am I misreading the gigantic comment in include/linux/cpumask.h? As far 
as I can tell, only x86 selects it and it is not user configurable 
unless CONFIG_DEBUG_PER_CPU_MAPS is set, which is buried in the debug 
options so most people won't bother trying to enable it. If I understand 
correctly, how should these be dealt with in the case of 
CONFIG_CPUMASK_OFFSTACK=n?

Cheers,
Nathan