Re: [PATCH v3 2/2] util/coroutine: Reduce default idle pool max size

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

On Tue, Oct 13, 2026 at 05:01:01PM -0500, Wesley Hershberger wrote:
> If g_file_get_contents fails (AppArmor, etc), use a more conservative
> default max size for the idle coroutine pool.
> 
> This avoids any possibility of the failure described in 86a637e48104
> unless the host kernel is configured with a lower-than-default
> vm.max_map_count.
> 
> This change is motivated by a discussion of backport risk:
> https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/2168803/comments/8
> 
> Fixes: 86a637e48104 ("coroutine: cap per-thread local pool size")
> Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Wesley Hershberger <wesley.hershberger@canonical.com>
> ---
>  util/qemu-coroutine.c | 75 ++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 48 insertions(+), 27 deletions(-)
> 
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 5af61995437..8ced93144fa 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -361,42 +361,63 @@ void qemu_coroutine_dec_pool_size(unsigned int removing_pool_size)
>      global_pool_max_size -= removing_pool_size;
>  }
>  
> -static unsigned int get_global_pool_hard_max_size(void)
> -{
>  #ifdef __linux__
> -    g_autofree char *contents = NULL;
> -    int max_map_count;
> +/*
> + * include/linux/mm.h
> + */
> +#define DEFAULT_MAX_MAP_COUNT 65530
>  
> -    /*
> -     * 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)) {
> +static unsigned int get_max_map_count(void)
> +{
> +    g_autofree char *contents = NULL;
> +    int max_map_count = 0;
> +    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents,
> +                            NULL, NULL)) {
>          /* contents has trailing newline, remove it before conversion */
>          g_strchomp(contents);
> -        if (qemu_strtoi(contents, NULL, 10, &max_map_count) != 0) {
> -            return UINT_MAX;
> -        }
> -        /*
> -         * This is an upper bound that avoids exceeding max_map_count. Leave a
> -         * fixed amount for non-coroutine users like library dependencies,
> -         * vhost-user, etc. Each coroutine takes up 2 VMAs so halve the
> -         * remaining amount.
> -         */
> -        if (max_map_count > 5000) {
> -            return (max_map_count - 5000) / 2;
> -        } else {
> -            /* Disable the global pool but threads still have local pools */
> -            return 0;
> +        if (qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
> +            return max_map_count;
>          }
>      }
> -#endif
> +    return 0;
> +}

I'm now realizing we could just return DEFAULT_MAX_MAP_COUNT
here and removed the conditional in the caller.

>  
> +/*
> + * 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.
> + */
> +static unsigned int get_global_pool_hard_max_size(void)
> +{
> +    /*
> +     * Read from /proc might fail due to mandatory access control (AppArmor,
> +     * SELinux, etc). Use the kernel's default in that case.
> +     */
> +    int max_map_count = get_max_map_count();
> +    if (!max_map_count) {
> +        max_map_count = DEFAULT_MAX_MAP_COUNT;
> +    }
> +
> +    /*
> +     * This is an upper bound that avoids exceeding max_map_count. Leave a
> +     * fixed amount for non-coroutine users like library dependencies,
> +     * vhost-user, etc. Each coroutine takes up 2 VMAs so halve the
> +     * remaining amount.
> +     */
> +    if (max_map_count > 5000) {
> +        return (max_map_count - 5000) / 2;
> +    } else {
> +        /* Disable the global pool but threads still have local pools */
> +        return 0;
> +    }
> +}
> +#else
> +static unsigned int get_global_pool_hard_max_size(void)
> +{
>      return UINT_MAX;
>  }
> +#endif
>  
>  static void __attribute__((constructor)) qemu_coroutine_init(void)
>  {
> 
> -- 
> 2.53.0
> 

With regards,
Daniel
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