Re: [PATCH v2 2/2] util/coroutine: Reduce default idle pool max size

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

On Tue, Oct 13, 2026 at 10:21:59AM -0500, Wesley Hershberger wrote:
> If g_file_get_contents fails (AppArmor or other LSM), 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 low 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")
> Signed-off-by: Wesley Hershberger <wesley.hershberger@canonical.com>
> ---
>  util/qemu-coroutine.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 5af61995437..cfe10c033e4 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -366,6 +366,11 @@ static unsigned int get_global_pool_hard_max_size(void)
>  #ifdef __linux__
>      g_autofree char *contents = NULL;
>      int max_map_count;
> +    /*
> +     * Read from /proc might fail due to LSM; use (DEFAULT_MAX_MAP_COUNT - 5000)
> +     * for distributions that don't change the kernel default
> +     */
> +    int default_pool_hard_max_size = 60530 / 2;
>  
>      /*
>       * Linux processes can have up to max_map_count virtual memory areas
> @@ -378,7 +383,7 @@ static unsigned int get_global_pool_hard_max_size(void)
>          /* contents has trailing newline, remove it before conversion */
>          g_strchomp(contents);
>          if (qemu_strtoi(contents, NULL, 10, &max_map_count) != 0) {
> -            return UINT_MAX;
> +            return default_pool_hard_max_size;
>          }
>          /*
>           * This is an upper bound that avoids exceeding max_map_count. Leave a
> @@ -395,7 +400,7 @@ static unsigned int get_global_pool_hard_max_size(void)
>      }
>  #endif
>  
> -    return UINT_MAX;
> +    return default_pool_hard_max_size;

This code is outside the __linux__ condition, but this is now applying
a Linux derived limit to it.

I'd suggest splitting the methods so it is more like this:

#ifdef __linux__
static unsigned int get_max_map_count(void)
{
    g_autofree char *contents = NULL;
    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents,
                            NULL, NULL)) {
        g_strchomp(contents);
        if (qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
            return max_map_count;
        }
    }
    return 0;
}

static unsigned int get_global_pool_hard_max_size(void)
{
    int max_map_count = get_max_map_count();
    if (!max_map_count) {
        max_map_count = 65530;
    }
    ...snip comments...
    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

With regards,
Daniel
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