Re: [PATCH 1/3] powerpc/kvm/book3s: Replace current->mm by kvm->mm

[Paul Mackerras <paulus@ozlabs.org>]

On Mon, Sep 23, 2019 at 06:24:07PM -0300, Leonardo Bras wrote:
> Given that in kvm_create_vm() there is:
> kvm->mm = current->mm;
> 
> And that on every kvm_*_ioctl we have:
> if (kvm->mm != current->mm)
> 	return -EIO;
> 
> I see no reason to keep using current->mm instead of kvm->mm.
> 
> By doing so, we would reduce the use of 'global' variables on code, relying
> more in the contents of kvm struct.

This patch led to a crash on shutting down a VM, because of this hunk:

> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index c4b606fe73eb..8069b35f2905 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -255,7 +255,7 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
>  
>  	kvm_put_kvm(stt->kvm);
>  
> -	account_locked_vm(current->mm,
> +	account_locked_vm(kvm->mm,
>  		kvmppc_stt_pages(kvmppc_tce_pages(stt->size)), false);

You are referencing kvm->mm after having done kvm_put_kvm a couple of
lines earlier, which means that *kvm can be freed at the point where
you use kvm->mm.  If you want to make this change you will need to
move the kvm_put_kvm call to after the last use of it.

I have dropped this patch for now.

Paul.