Re: [Qemu-devel] [PATCH 01/15] coroutine-ucontext: mmap stack memory

[Paolo Bonzini]

On 28/06/2016 11:01, Peter Lieven wrote:
> +#ifdef MAP_GROWSDOWN
> +    co->stack = mmap(NULL, COROUTINE_STACK_SIZE, PROT_READ | PROT_WRITE,
> +                     MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN, -1, 0);
> +    if (co->stack == MAP_FAILED) {
> +        abort();
> +    }
> +    /* add a guard page at bottom of the stack */
> +    if (mmap(co->stack, getpagesize(), PROT_NONE,
> +        MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN, -1, 0) == MAP_FAILED) {
> +        abort();
> +    }

Nevermind, you added a guard page!  Good! :)  And actually it looks like
the stack usage has been mostly tamed, at least for the block layer.

On the other hand MAP_GROWSDOWN automatically adds a guard page since
Linux 3.9 (see commit 09884964335e, "mm: do not grow the stack vma just
because of an overrun on preceding vma", 2013-02-27), so as it turns out
you don't even need the guard page!

Paolo

> +#else
>      co->stack = g_malloc(stack_size);
> +#endif