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SubjectRe: Light-weight dynamically extended stacks
On Sun, Dec 18, 2005 at 04:12:49PM -0800, Matt Mackall wrote:

> Perhaps the time for this has come and gone, but it occurred to me
> that it should be relatively straightforward to make a form of
> dynamically extended stacks that are appropriate to the kernel.
>
> While we have a good handle on most of the worst stack offenders, we
> can still run into trouble with pathological cases (say, symlink
> recursion for XFS on a RAID built from loopback mounts over NFS
> tunneled over IPSEC through GRE). So there's probably no
> one-size-fits-all when it comes to stack size.

My count of bug reports for problems with in-kernel code with 4k stacks
after Neil's patch went into -mm is still at 0. That's amazing
considering how many people have claimed in this thread how unstable
4k stacks were...

> Rather than relying on guard pages and VM faults like userspace, we
> can use a cooperative scheme where we "label" call paths that might be
> extra deep (recursion through the block layer, network tunnels,
> symlinks, etc.) with something like the following:
>
> ret = grow_stack(function, arg, GFP_ATOMIC);
>
> This is much like cond_resched() except for stack usage rather than
> CPU usage. grow_stack() checks if we're in the danger zone for stack
> usage (say 1k remaining), and if so, allocates a new stack and
> swizzles the stack pointer over to it.
>...

If more than 3 kB of stack is used on i386 that's a bug.
And we should fix bugs, not work around them.

CONFIG_DEBUG_STACKOVERFLOW in arch/i386/kernel/irq.c shows the correct
approach:
- a debug option for not harming performance
- dump_stack() when too much stack is used

cu
Adrian

--

"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed

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