linux-kernel - Re: [PATCH 1/4] KVM: PPC: Book3S HV: Drop prepare_done from struct kvm_resize

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Message-ID: <20171130034040.GO3023@umbus.fritz.box>
Date:   Thu, 30 Nov 2017 14:40:40 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Serhii Popovych <spopovyc@...hat.com>
Cc:     linux-kernel@...r.kernel.org, michael@...erman.id.au,
        paulus@...ba.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-ppc@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: PPC: Book3S HV: Drop prepare_done from struct
 kvm_resize_hpt and cleanups

On Wed, Nov 29, 2017 at 11:38:23AM -0500, Serhii Popovych wrote:
> Replace ->prepare_done flag functionality with special handling
> of -EBUSY in ->error as indicator that allocation work is running.
> 
> Besides cosmetics this reduces size of struct kvm_resize_hpt by
> __alignof__(struct kvm_hpt_info) and saves few bytes of code.
> 
> While there correct comment in struct kvm_resize_hpt about locking
> used to protect access to certain fields.
> 
> Assert with BUG_ON() in case of HPT allocation thread work runs
> more than once for resize request or resize_hpt_allocate()
> returns -EBUSY that is treated specially.
> 
> Change comparison against zero to make checkpatch.pl happy.
> 
> Signed-off-by: Serhii Popovych <spopovyc@...hat.com>

Reviewed-by: David Gibson <david@...son.dropbear.id.au>

> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 42 ++++++++++++++++++++++---------------
>  1 file changed, 25 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 235319c..0534aab 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -65,11 +65,17 @@ struct kvm_resize_hpt {
>  	u32 order;
>  
>  	/* These fields protected by kvm->lock */
> +
> +	/* Possible values and their usage:
> +	 *  <0     an error occurred during allocation,
> +	  * -EBUSY allocation is in the progress,
> +	 *  0      allocation made successfuly.
> +	 */
>  	int error;
> -	bool prepare_done;
>  
> -	/* Private to the work thread, until prepare_done is true,
> -	 * then protected by kvm->resize_hpt_sem */
> +	/* Private to the work thread, until error != -EBUSY,
> +	 * then protected by kvm->lock.
> +	 */
>  	struct kvm_hpt_info hpt;
>  };
>  
> @@ -1432,15 +1438,21 @@ static void resize_hpt_prepare_work(struct work_struct *work)
>  	struct kvm *kvm = resize->kvm;
>  	int err;
>  
> +	BUG_ON(resize->error != -EBUSY);
> +
>  	resize_hpt_debug(resize, "resize_hpt_prepare_work(): order = %d\n",
>  			 resize->order);
>  
>  	err = resize_hpt_allocate(resize);
>  
> +	/* We have strict assumption about -EBUSY
> +	 * when preparing for HPT resize.
> +	 */
> +	BUG_ON(err == -EBUSY);
> +
>  	mutex_lock(&kvm->lock);
>  
>  	resize->error = err;
> -	resize->prepare_done = true;
>  
>  	mutex_unlock(&kvm->lock);
>  }
> @@ -1465,14 +1477,12 @@ long kvm_vm_ioctl_resize_hpt_prepare(struct kvm *kvm,
>  
>  	if (resize) {
>  		if (resize->order == shift) {
> -			/* Suitable resize in progress */
> -			if (resize->prepare_done) {
> -				ret = resize->error;
> -				if (ret != 0)
> -					resize_hpt_release(kvm, resize);
> -			} else {
> +			/* Suitable resize in progress? */
> +			ret = resize->error;
> +			if (ret == -EBUSY)
>  				ret = 100; /* estimated time in ms */
> -			}
> +			else if (ret)
> +				resize_hpt_release(kvm, resize);
>  
>  			goto out;
>  		}
> @@ -1492,6 +1502,8 @@ long kvm_vm_ioctl_resize_hpt_prepare(struct kvm *kvm,
>  		ret = -ENOMEM;
>  		goto out;
>  	}
> +
> +	resize->error = -EBUSY;
>  	resize->order = shift;
>  	resize->kvm = kvm;
>  	INIT_WORK(&resize->work, resize_hpt_prepare_work);
> @@ -1546,16 +1558,12 @@ long kvm_vm_ioctl_resize_hpt_commit(struct kvm *kvm,
>  	if (!resize || (resize->order != shift))
>  		goto out;
>  
> -	ret = -EBUSY;
> -	if (!resize->prepare_done)
> -		goto out;
> -
>  	ret = resize->error;
> -	if (ret != 0)
> +	if (ret)
>  		goto out;
>  
>  	ret = resize_hpt_rehash(resize);
> -	if (ret != 0)
> +	if (ret)
>  		goto out;
>  
>  	resize_hpt_pivot(resize);

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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