linux 资源限制，软限制可以超出硬限制么？

while(f > oldlimit) {
            int decr_step = 16;

            limit.rlim_cur = f;
            limit.rlim_max = f;
            if (setrlimit(RLIMIT_NOFILE,&limit) != -1) break;
            setrlimit_error = errno;

            /* We failed to set file limit to 'f'. Try with a
             * smaller limit decrementing by a few FDs per iteration. */
            if (f < decr_step) break;
            f -= decr_step;
        }

网上说软限制不能超出硬限制，为什么不直接把软限制改成和硬限制一样大呢，如上代码（redis源码）还尝试什么？