linux-kernel - Re: [PATCH] vfio/type1: Limit DMA mappings per container

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Date:   Tue, 2 Apr 2019 10:41:15 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        eric.auger@...hat.com, cohuck@...hat.com
Subject: Re: [PATCH] vfio/type1: Limit DMA mappings per container

On Mon, Apr 01, 2019 at 02:16:52PM -0600, Alex Williamson wrote:

[...]

> @@ -1081,8 +1088,14 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
>  		goto out_unlock;
>  	}
>  
> +	if (!atomic_add_unless(&iommu->dma_avail, -1, 0)) {
> +		ret = -ENOSPC;
> +		goto out_unlock;
> +	}
> +
>  	dma = kzalloc(sizeof(*dma), GFP_KERNEL);
>  	if (!dma) {
> +		atomic_inc(&iommu->dma_avail);

This should be the only special path to revert the change.  Not sure
whether this can be avoided by simply using atomic_read() or even
READ_ONCE() (I feel like we don't need atomic ops with dma_avail
because we've had the mutex but it of course it doesn't hurt...) to
replace atomic_add_unless() above to check against zero then we do
+1/-1 in vfio_[un]link_dma() only.  But AFAICT this patch is correct.

Reviewed-by: Peter Xu <peterx@...hat.com>

Thanks,

-- 
Peter Xu