linux-kernel - Re: [PATCH v2 06/12] iommu: Make dev->fault

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Message-ID: <ZNUqV5Mte2AsVa1L@ziepe.ca>
Date:   Thu, 10 Aug 2023 15:20:07 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/12] iommu: Make dev->fault_param static

On Thu, Jul 27, 2023 at 01:48:31PM +0800, Lu Baolu wrote:
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 4ba3bb692993..3e4ff984aa85 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -302,7 +302,15 @@ static int dev_iommu_get(struct device *dev)
>  		return -ENOMEM;
>  
>  	mutex_init(&param->lock);
> +	param->fault_param = kzalloc(sizeof(*param->fault_param), GFP_KERNEL);
> +	if (!param->fault_param) {
> +		kfree(param);
> +		return -ENOMEM;
> +	}
> +	mutex_init(&param->fault_param->lock);
> +	INIT_LIST_HEAD(&param->fault_param->faults);
>  	dev->iommu = param;

This allocation seems pointless?

If we always allocate the fault param then just don't make it a
pointer in the first place.

The appeal of allocation would be to save a few bytes in the common
case that the driver doesn't support faulting.

Which means the driver needs to make some call to enable faulting for
a device. In this case I'd continue to lazy free on release like this
patch does.

Jason