On Tue, Sep 07, 2021 at 04:14:24PM -0700, Linus Torvalds wrote:

[ Added maintainers for various bits and pieces, since I spent the
time trying to look at why those bits and pieces wasted stack-space
and caused problems ]

On Tue, Sep 7, 2021 at 3:16 PM Linus Torvalds
[off-list ref] 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.

A lot of the stack frame size warnings are hidden by the fact that our
default value for warning about stack usage is 2kB for 64-bit builds.

Probably exactly because people did things like that cpumask thing,
and have these arrays of structures that are often even bigger in the
64-bit world.

Thanks for the heads-up. I found one instance of this bad practice in
hv_apic.c. Presumably that's the one you were referring to.

However calling into the allocator from that IPI path seems very heavy
weight. I will discuss with fellow engineers on how to fix it properly.

Wei.

                Linus