linux-kernel - Re: [PATCH] kvm: no need to check return value of debugfs

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Date:   Tue, 22 Jan 2019 21:39:24 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH] kvm: no need to check return value of debugfs_create
 functions



On 22.01.2019 16:21, Greg Kroah-Hartman wrote:
> When calling debugfs functions, there is no need to ever check the
> return value.  The function can work or not, but the code logic should
> never do something different based on this.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> Cc: kvm@...r.kernel.org
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  virt/kvm/kvm_main.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5ecea812cb6a..4f96450ecdfc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2528,9 +2528,7 @@ static int kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
> 
>  	snprintf(dir_name, sizeof(dir_name), "vcpu%d", vcpu->vcpu_id);
>  	vcpu->debugfs_dentry = debugfs_create_dir(dir_name,
> -								vcpu->kvm->debugfs_dentry);
> -	if (!vcpu->debugfs_dentry)
> -		return -ENOMEM;
> +						  vcpu->kvm->debugfs_dentry);
> 
>  	ret = kvm_arch_create_vcpu_debugfs(vcpu);
>  	if (ret < 0) {
> 


The interesting part of these debugfs entries is that they export an interface that is used
by the kvm_stat tool. (and all distributions that I checked have debugfs enabled).

I think it is pretty unlikely that things will fail, but the question is: do we want to reject
VM creation if that VM cannot be observed by instrumentation or not? No idea.

This also brings the question: shall we move these counters out of debugfs into something else?

Christian