Re: [PATCH] KVM: MMU: fix an IS_ERR() vs NULL bug

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 01, 2022, Dan Carpenter wrote:
> The alloc_workqueue() function does not return error pointers, it
> returns NULL on error.  Update the check accordingly.
> 
> Fixes: 1a3320dd2939 ("KVM: MMU: propagate alloc_workqueue failure")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

> Obviously, I noticed that the patch says "propagate alloc_workqueue
> failure" so that's a puzzling thing.  Merge issue perhaps?  In
> linux-next it alloc_workqueue() returns NULL.

No merge issue, just a goof.  The "propagate" patch was added because KVM neglected
to check for allocation failure, so at least it was a step in the right direction :-)

>  arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index a2f9a34a0168..d71d177ae6b8 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -22,8 +22,8 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
>  		return 0;
>  
>  	wq = alloc_workqueue("kvm", WQ_UNBOUND|WQ_MEM_RECLAIM|WQ_CPU_INTENSIVE, 0);
> -	if (IS_ERR(wq))
> -		return PTR_ERR(wq);
> +	if (!wq)
> +		return -ENOMEM;
>  
>  	/* This should not be changed for the lifetime of the VM. */
>  	kvm->arch.tdp_mmu_enabled = true;
> -- 

Paolo, any objection to also returning '0' in all non-error paths?  There's no
need to return whether or not the TDP MMU is enabled since that's handled locally,
and the "return 1" is rather odd.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index dbf46dd98618..dec32b4a13aa 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5779,7 +5779,7 @@ int kvm_mmu_init_vm(struct kvm *kvm)
        spin_lock_init(&kvm->arch.mmu_unsync_pages_lock);

        r = kvm_mmu_init_tdp_mmu(kvm);
-       if (r < 0)
+       if (r)
                return r;

        node->track_write = kvm_mmu_pte_write;
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index a2f9a34a0168..3a60b999e1aa 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -31,7 +31,7 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
        spin_lock_init(&kvm->arch.tdp_mmu_pages_lock);
        INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
        kvm->arch.tdp_mmu_zap_wq = wq;
-       return 1;
+       return 0;
 }

 /* Arbitrarily returns true so that this may be used in if statements. */