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 torvalds@linux-foundation.org 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