Re: [PATCH] coroutine: cap per-thread local pool size

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 18, 2024 at 02:34:29PM -0400, Stefan Hajnoczi wrote:
> The coroutine pool implementation can hit the Linux vm.max_map_count
> limit, causing QEMU to abort with "failed to allocate memory for stack"
> or "failed to set up stack guard page" during coroutine creation.
> 
> This happens because per-thread pools can grow to tens of thousands of
> coroutines. Each coroutine causes 2 virtual memory areas to be created.

This sounds quite alarming. What usage scenario is justified in
creating so many coroutines ?

IIUC, coroutine stack size is 1 MB, and so tens of thousands of
coroutines implies 10's of GB of memory just on stacks alone.

> Eventually vm.max_map_count is reached and memory-related syscalls fail.

On my system max_map_count is 1048576, quite alot higher than
10's of 1000's. Hitting that would imply ~500,000 coroutines and
~500 GB of stacks !

> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 5fd2dbaf8b..2790959eaf 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c

> +static unsigned int get_global_pool_hard_max_size(void)
> +{
> +#ifdef __linux__
> +    g_autofree char *contents = NULL;
> +    int max_map_count;
> +
> +    /*
> +     * Linux processes can have up to max_map_count virtual memory areas
> +     * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond this limit. We
> +     * must limit the coroutine pool to a safe size to avoid running out of
> +     * VMAs.
> +     */
> +    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, NULL,
> +                            NULL) &&
> +        qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
> +        /*
> +         * This is a conservative upper bound that avoids exceeding
> +         * max_map_count. Leave half for non-coroutine users like library
> +         * dependencies, vhost-user, etc. Each coroutine takes up 2 VMAs so
> +         * halve the amount again.
> +         */
> +        return max_map_count / 4;

That's 256,000 coroutines, which still sounds incredibly large
to me.

> +    }
> +#endif
> +
> +    return UINT_MAX;

Why UINT_MAX as a default ?  If we can't read procfs, we should
assume some much smaller sane default IMHO, that corresponds to
what current linux default max_map_count would be.

> +}
> +
> +static void __attribute__((constructor)) qemu_coroutine_init(void)
> +{
> +    qemu_mutex_init(&global_pool_lock);
> +    global_pool_hard_max_size = get_global_pool_hard_max_size();
>  }
> -- 
> 2.44.0
> 
> 

With regards,
Daniel
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