Re: [RFC 2/2] x86_64: expand kernel stack to 16K

[Dave Chinner]

On Thu, May 29, 2014 at 08:24:49AM -0700, Linus Torvalds wrote:
> On Thu, May 29, 2014 at 12:26 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >
> > What concerns me about both __alloc_pages_nodemask() and
> > kernel_map_pages is that when I look at the code I see functions
> > that have no obvious stack usage problem. However, the compiler is
> > producing functions with huge stack footprints and it's not at all
> > obvious when I read the code. So in this case I'm more concerned
> > that we have a major disconnect between the source code structure
> > and the code that the compiler produces...
> 
> I agree. In fact, this is the main reason that Minchan's call trace
> and this thread has actually convinced me that yes, we really do need
> to make x86-64 have a 16kB stack (well, 16kB allocation - there's
> still the thread info etc too).
> 
> Usually when we see the stack-smashing traces, they are because
> somebody did something stupid. In this case, there are certainly
> stupid details, and things I think we should fix, but there is *not*
> the usual red flag of "Christ, somebody did something _really_ wrong".
> 
> So I'm not in fact arguing against Minchan's patch of upping
> THREAD_SIZE_ORDER to 2 on x86-64, but at the same time stack size does
> remain one of my "we really need to be careful" issues, so while I am
> basically planning on applying that patch, I _also_ want to make sure
> that we fix the problems we do see and not just paper them over.
> 
> The 8kB stack has been somewhat restrictive and painful for a while,
> and I'm ok with admitting that it is just getting _too_ damn painful,
> but I don't want to just give up entirely when we have a known deep
> stack case.

That sounds like a plan. Perhaps it would be useful to add a
WARN_ON_ONCE(stack_usage > 8k) (or some other arbitrary depth beyond
8k) so that we get some indication that we're hitting a deep stack
but the system otherwise keeps functioning. That gives us some
motivation to keep stack usage down but isn't a fatal problem like
it is now....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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