[PATCH 23/24] powerpc/mm: Cleanup check for stack expansion

[Michael Ellerman]

LEROY Christophe <christophe.leroy at c-s.fr> writes:

> Michael Ellerman <mpe at ellerman.id.au> a écrit :
>
>> LEROY Christophe <christophe.leroy at c-s.fr> writes:
>>
>>> Benjamin Herrenschmidt <benh at kernel.crashing.org> a écrit :
>>>
>>>> When hitting below a VM_GROWSDOWN vma (typically growing the stack),
>>>> we check whether it's a valid stack-growing instruction and we
>>>> check the distance to GPR1. This is largely open coded with lots
>>>> of comments, so move it out to a helper.
>>>
>>> Did you have a look at the following patch ? It's been waiting for
>>> application for some weeks now.
>>> https://patchwork.ozlabs.org/patch/771869
>>
>> I actually merged it last merge window, but found I had no good way to
>> test it, so I took it out again until I can write a test case for it.
>>
>> The way I realised it wasn't being tested was by removing all the
>> store_updates_sp logic entirely and having my system run happily for
>> several days :}
>
> Which demonstrates how unlikely this is, hence doing that get_user()  
> at every fault is waste of time.

Yes I agree.

> How do you plan to handle that in parralele to ben's serie ?

Not sure :)

> I'll be back from vacation next week and may help finding a way to  
> test that. (A test program using alloca() ?)

I was thinking hand-crafted asm, but that might be a pain to get working
for 32 & 64-bit, in which case alloca() might work.

cheers