> On Sep 20, 2018, at 8:12 PM, Andrew Lunn <andrew@xxxxxxx> wrote: > >> On Fri, Sep 21, 2018 at 02:11:43AM +0200, Jason A. Donenfeld wrote: >> Hey Arnd, >> >>> On Thu, Sep 20, 2018 at 6:02 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: >>> Right, if you hit a stack requirement like this, it's usually the compiler >>> doing something bad, not just using too much stack but also generating >>> rather slow object code in the process. It's better to fix the bug by >>> optimizing the code to not spill registers to the stack. >>> >>> In the long run, I'd like to reduce the stack frame size further, so >>> best assume that anything over 1024 bytes (on 32-bit) or 1280 bytes >>> (on 64-bit) is a bug in the code, and stay below that. >>> >>> For prototyping, you can just mark the broken functions individually >>> by setting the warning limit for a specific function that is known to >>> be misoptimized by the compiler (with a comment about which compiler >>> and architectures are affected), but not override the limit for the >>> entire file. >> >> Thanks for the explanation. Fortunately in my case, the issues were >> trivially fixable to get it under 1024/1280. (By the way, why does >> 64-bit have a slightly larger stack frame? To account for 32 pointers >> taking double the space or something?) That will be rectified in v6. > > Hi Jason > > Do you any stack usage information? > > A VPN can be at the bottom of some deep stack calls. Swap on NFS over > the VPN? If you have one frame of 1K, you might be O.K. But if you > have a few of these, i can see there might be issues of overflowing > the stack. > > At the risk on suggesting something awful: on x86_64, since we turn preemption off for simd, it wouldn’t be *completely* insane to do the crypto on the irq stack. It would look like: kernel_fpu_call(func, arg); And this helper would disable preemption, enable FPU, switch to the irq stack, call func(arg), disable FPU, enable preemption, and return. And we can have large IRQ stacks. I refuse to touch this with a ten-foot pole until the lazy FPU restore patches land. All that being said, why are these frames so large? It sounds like something may be spilling that ought not to.
