Re: [PATCH v5 00/32] virtually mapped stacks and thread_info cleanup

[Christian Borntraeger]

On 07/13/2016 08:36 PM, Andy Lutomirski wrote:
> On Wed, Jul 13, 2016 at 1:54 AM, Christian Borntraeger
> <borntraeger@xxxxxxxxxx> wrote:
>> On 07/11/2016 10:53 PM, Andy Lutomirski wrote:
>>> Hi all-
>>>
>>> Since the dawn of time, a kernel stack overflow has been a real PITA
>>> to debug, has caused nondeterministic crashes some time after the
>>> actual overflow, and has generally been easy to exploit for root.
>>>
>>> With this series, arches can enable HAVE_ARCH_VMAP_STACK.  Arches
>>> that enable it (just x86 for now) get virtually mapped stacks with
>>> guard pages.  This causes reliable faults when the stack overflows.
>>>
>>> If the arch implements it well, we get a nice OOPS on stack overflow
>>> (as opposed to panicing directly or otherwise exploding badly).  On
>>> x86, the OOPS is nice, has a usable call trace, and the overflowing
>>> task is killed cleanly.
>>>
>>> This series (starting with v4) also extensively cleans up
>>> thread_info.  thread_info has been partially redundant with
>>> thread_struct for a long time -- both are places for arch code to
>>> add additional per-task variables.  thread_struct is much cleaner:
>>> it's always in task_struct, and there's nothing particularly magical
>>> about it.  So this series contains a bunch of cleanups on x86 to
>>> move almost everything from thread_info to thread_struct (which,
>>> even by itself, deletes more code than it adds) and to remove x86's
>>> dependence on thread_info's position on the stack.  Then it opts x86
>>> into a new config option THREAD_INFO_IN_TASK to get rid of
>>> arch-specific thread_info entirely and simply embed a defanged
>>> thread_info (containing only flags) and 'int cpu' into task_struct.
>>>
>>> Once thread_info stops being magical, there's another benefit: we
>>> can free the thread stack as soon as the task is dead (without
>>> waiting for RCU) and then, if vmapped stacks are in use, cache the
>>> entire stack for reuse on the same cpu.
>>>
>>> This seems to be an overall speedup of about 0.5-1 Âs per
>>> pthread_create/join in a simple test -- a percpu cache of vmalloced
>>> stacks appears to be a bit faster than a high-order stack
>>> allocation, at least when the cache hits.  (I expect that workloads
>>> with a low cache hit rate are likely to be dominated by other
>>> effects anyway.)
>>>
>>> This does not address interrupt stacks.
>>>
>>> It's worth noting that s390 has an arch-specific gcc feature that
>>> detects stack overflows by adjusting function prologues.  Arches
>>> with features like that may wish to avoid using vmapped stacks to
>>> minimize the performance hit.
>>
>> Yes, might not need this for stack overflow detection. What might
>> be interesting is the thread_info/thread_struct change, if we can
>> strip down thread_info.(CONFIG_THREAD_INFO_IN_TASK). Would it actually
>> make sense to separate these two changes to see what performance
>> impact  CONFIG_THREAD_INFO_IN_TASK has on its own?
>>
> 
> They're already separated.
> 
> CONFIG_THREAD_INFO_IN_TASK should have basically no performance impact
> unless there are arch-dependent (percpu?) issues involved.  It does
> enable immediate thread stack deallocation, though, and it would be
> straightforward to make CONFIG_THREAD_INFO_IN_TASK cache stacks even
> if CONFIG_VMAP_STACK=n.  That should be a moderate clone() speedup.

Yes. My point was more of having two patch series in case the discussion goes
on regarding virtual stack.