runtime: grow stack by copying

On stack overflow, if all frames on the stack are
copyable, we copy the frames to a new stack twice
as large as the old one.  During GC, if a G is using
less than 1/4 of its stack, copy the stack to a stack
half its size.

TODO
- Do something about C frames.  When a C frame is in the
  stack segment, it isn't copyable.  We allocate a new segment
  in this case.
  - For idempotent C code, we can abort it, copy the stack,
    then retry.  I'm working on a separate CL for this.
  - For other C code, we can raise the stackguard
    to the lowest Go frame so the next call that Go frame
    makes triggers a copy, which will then succeed.
- Pick a starting stack size?

The plan is that eventually we reach a point where the
stack contains only copyable frames.

LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/54650044

1 files changed, 6 insertions, 0 deletions

diff --git a/src/pkg/runtime/mem_linux.c b/src/pkg/runtime/mem_linux.c
index 2786ad70f6..bab05011ff 100644
--- a/src/pkg/runtime/mem_linux.c
+++ b/src/pkg/runtime/mem_linux.c
@@ -92,6 +92,12 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 	runtime·munmap(v, n);
 }
 
+void
+runtime·SysFault(void *v, uintptr n)
+{
+	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+}
+
 void*
 runtime·SysReserve(void *v, uintptr n)
 {