[RFC v3] fiber: Increase default stack size

[Vladimir Davydov]

On Tue, Feb 26, 2019 at 02:16:32PM +0300, Cyrill Gorcunov wrote:
> >  - 1 MB for max stack size seems to be a bit of an overkill for now.
> >    The default value should be set to 256 KB, but we do need a
> >    configuration option for it. Let's add it to the fiber Lua module.
> >    May be done in a separate patch, but should be submitted together in
> >    the same patch set.
> 
> Wait, first fiber for main cord is created before lua init, isn't it?
> I already though about using lua config for it but fiber init'ed at
> very early stage.

Yeah, good point, I haven't thought about it. Then we have to either
always allocate a big stack for the main fiber or use an environment
variable. Let's get back to it later and first make the stack size limit
statically defined.

> 
> >  - 16 byte unique identifier for detecting stack overflow doesn't seem
> >    to be enough. Imagine a PATH_MAX buffer allocated on stack that uses
> >    only a hundred bytes for path formatting. It can easily jump over the
> >    watermark. We should probably use random poisoning: say, 4 unique
> >    identifiers 8 bytes each scattered a few hundred bytes apart. Some
> >    math/reasoning behind this would be nice to see in the comments.
> 
> If we want to scatter we should simply put marks at page bounds.
> Dirtifying somewhere inside middle of a page is useless.

Hmm, why? Consider the example with PATH_MAX buffer. Putting dirty marks
at page boundaries doesn't guarantee any of them will get overwritten by
the buffer if only a few hundred of bytes are used. I think we should
dirty the last page or two at random intervals - this should increase
the chance that at least one mark is overwritten by any function that is
eager for the stack.