linux-kernel - Re: [PATCH] KVM: selftests: Make sure kvm_create_max_vcpus test won't hit RLIMIT

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Date:   Mon, 22 Nov 2021 17:38:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: Make sure kvm_create_max_vcpus test
 won't hit RLIMIT_NOFILE

On Mon, Nov 22, 2021, Vitaly Kuznetsov wrote:
> With the elevated 'KVM_CAP_MAX_VCPUS' value kvm_create_max_vcpus test
> may hit RLIMIT_NOFILE limits:
> 
>  # ./kvm_create_max_vcpus
>  KVM_CAP_MAX_VCPU_ID: 4096
>  KVM_CAP_MAX_VCPUS: 1024
>  Testing creating 1024 vCPUs, with IDs 0...1023.
>  /dev/kvm not available (errno: 24), skipping test
> 
> Adjust RLIMIT_NOFILE limits to make sure KVM_CAP_MAX_VCPUS fds can be
> opened. Note, raising hard limit ('rlim_max') requires CAP_SYS_RESOURCE
> capability which is generally not needed to run kvm selftests (but without
> raising the limit the test is doomed to fail anyway).
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  .../selftests/kvm/kvm_create_max_vcpus.c      | 22 +++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/kvm_create_max_vcpus.c b/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
> index f968dfd4ee88..19198477a10e 100644
> --- a/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
> +++ b/tools/testing/selftests/kvm/kvm_create_max_vcpus.c
> @@ -12,6 +12,7 @@
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
> +#include <sys/resource.h>
>  
>  #include "test_util.h"
>  
> @@ -19,6 +20,9 @@
>  #include "asm/kvm.h"
>  #include "linux/kvm.h"
>  
> +/* 'Safe' number of open file descriptors in addition to vCPU fds needed */
> +#define NOFD 16

Any reason not to make this "buffer" extra large, e.g. 100+ to avoid having to
debug this issue again in the future?

> +
>  void test_vcpu_creation(int first_vcpu_id, int num_vcpus)
>  {
>  	struct kvm_vm *vm;
> @@ -40,10 +44,28 @@ int main(int argc, char *argv[])
>  {
>  	int kvm_max_vcpu_id = kvm_check_cap(KVM_CAP_MAX_VCPU_ID);
>  	int kvm_max_vcpus = kvm_check_cap(KVM_CAP_MAX_VCPUS);

Rather than a separate define that's hard to describe succintly, what about:

	int nr_fds_wanted = kvm_max_vcpus + <arbitrary number>

and then the body becomes

	if (nr_fds_wanted > rl.rlim_cur) {
		rl.rlim_cur = nr_fds_wanted;
		rl.rlim_max = max(rl.rlim_max, nr_fds_wanted);

		...
	}

> +	struct rlimit rl;
>  
>  	pr_info("KVM_CAP_MAX_VCPU_ID: %d\n", kvm_max_vcpu_id);
>  	pr_info("KVM_CAP_MAX_VCPUS: %d\n", kvm_max_vcpus);
>  
> +	/*
> +	 * Creating KVM_CAP_MAX_VCPUS vCPUs require KVM_CAP_MAX_VCPUS open
> +	 * file decriptors.
> +	 */
> +	TEST_ASSERT(!getrlimit(RLIMIT_NOFILE, &rl),
> +		    "getrlimit() failed (errno: %d)", errno);

And strerror() output too?

> +
> +	if (kvm_max_vcpus > rl.rlim_cur - NOFD) {
> +		rl.rlim_cur = kvm_max_vcpus + NOFD;
> +
> +		if (kvm_max_vcpus > rl.rlim_max - NOFD)
> +			rl.rlim_max = kvm_max_vcpus + NOFD;
> +
> +		TEST_ASSERT(!setrlimit(RLIMIT_NOFILE, &rl),
> +			    "setrlimit() failed (errno: %d)", errno);
> +	}
> +
>  	/*
>  	 * Upstream KVM prior to 4.8 does not support KVM_CAP_MAX_VCPU_ID.
>  	 * Userspace is supposed to use KVM_CAP_MAX_VCPUS as the maximum ID
> -- 
> 2.33.1
>