Re: [PATCH 3/4] kernel/fork: switch vmapped stack callation to __vmalloc_area()

[Andy Lutomirski]

On Wed, Feb 21, 2018 at 7:23 AM, Konstantin Khlebnikov
<khlebnikov@xxxxxxxxxxxxxx> wrote:
>
>
> On 21.02.2018 03:16, Andrew Morton wrote:
>>
>> On Tue, 23 Jan 2018 16:57:21 +0300 Konstantin Khlebnikov
>> <khlebnikov@xxxxxxxxxxxxxx> wrote:
>>
>>> # stress-ng --clone 100 -t 10s --metrics-brief
>>> at 32-core machine shows boost 35000 -> 36000 bogo ops
>>>
>>> Patch 4/4 is a kind of RFC.
>>> Actually per-cpu cache of preallocated stacks works faster than buddy
>>> allocator thus
>>> performance boots for it happens only at completely insane rate of
>>> clones.
>>>
>>
>> I'm not really sure what to make of this patchset.  Is it useful in any
>> known real-world use cases?
>
>
> Not yet. Feel free to ignore last patch.
>
>>
>>> +         This option neutralize stack overflow protection but allows to
>>> +         achieve best performance for syscalls fork() and clone().
>>
>>
>> That sounds problematic, but perhaps acceptable if the fallback only
>> happens rarely.
>>
>> Can this code be folded into CONFIG_VMAP_STACk in some cleaner fashion?
>> We now have options for non-vmapped stacks, vmapped stacks and a mix
>> of both.
>>
>> And what about this comment in arch/Kconfig:VMAP_STACK:
>>
>>            This is presently incompatible with KASAN because KASAN expects
>>            the stack to map directly to the KASAN shadow map using a
>> formula
>>            that is incorrect if the stack is in vmalloc space.
>>
>>
>> So VMAP_STACK_AS_FALLBACK will intermittently break KASAN?
>>
>
> All of this (including CONFIG_VMAP_STACK) could be turned into boot option.
> I think this would be a best solution.

Or someone could *fix* KASAN to work with stacks in the vmalloc area.