linux-kernel - Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

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Message-ID: <20251017054007.GB6199@unreal>
Date: Fri, 17 Oct 2025 08:40:07 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	kvm@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linux-pci@...r.kernel.org, Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO
 regions

On Thu, Oct 16, 2025 at 08:53:32PM -0300, Jason Gunthorpe wrote:
> On Mon, Oct 13, 2025 at 06:26:11PM +0300, Leon Romanovsky wrote:
> > +
> > +static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> > +				   struct dma_buf_attachment *attachment)
> > +{
> > +	struct vfio_pci_dma_buf *priv = dmabuf->priv;
> > +
> > +	if (!attachment->peer2peer)
> > +		return -EOPNOTSUPP;
> > +
> > +	if (priv->revoked)
> > +		return -ENODEV;
> > +
> > +	switch (pci_p2pdma_map_type(priv->provider, attachment->dev)) {
> > +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> > +		break;
> > +	case PCI_P2PDMA_MAP_BUS_ADDR:
> > +		/*
> > +		 * There is no need in IOVA at all for this flow.
> > +		 * We rely on attachment->priv == NULL as a marker
> > +		 * for this mode.
> > +		 */
> > +		return 0;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> > +	attachment->priv = kzalloc(sizeof(struct dma_iova_state), GFP_KERNEL);
> > +	if (!attachment->priv)
> > +		return -ENOMEM;
> > +
> > +	dma_iova_try_alloc(attachment->dev, attachment->priv, 0, priv->size);
> 
> The lifetime of this isn't good..
> 
> > +	return 0;
> > +}
> > +
> > +static void vfio_pci_dma_buf_detach(struct dma_buf *dmabuf,
> > +				    struct dma_buf_attachment *attachment)
> > +{
> > +	kfree(attachment->priv);
> > +}
> 
> If the caller fails to call map then it leaks the iova.

I'm relying on dmabuf code and documentation:

   926 /**
   927  * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list
...   
   932  *
   933  * Returns struct dma_buf_attachment pointer for this attachment. Attachments
   934  * must be cleaned up by calling dma_buf_detach().

Successful call to vfio_pci_dma_buf_attach() MUST be accompanied by call
to vfio_pci_dma_buf_detach(), so as far as dmabuf implementation follows
it, there is no leak.

> 
> > +static struct sg_table *
> > +vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> > +		     enum dma_data_direction dir)
> > +{
> [..]
> 
> 
> > +err_unmap_dma:
> > +	if (!i || !state)
> > +		; /* Do nothing */
> > +	else if (dma_use_iova(state))
> > +		dma_iova_destroy(attachment->dev, state, mapped_len, dir,
> > +				 attrs);
> 
> If we hit this error path then it is freed..
> 
> > +static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> > +				   struct sg_table *sgt,
> > +				   enum dma_data_direction dir)
> > +{
> > +	struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> > +	struct dma_iova_state *state = attachment->priv;
> > +	unsigned long attrs = DMA_ATTR_MMIO;
> > +	struct scatterlist *sgl;
> > +	int i;
> > +
> > +	if (!state)
> > +		; /* Do nothing */
> > +	else if (dma_use_iova(state))
> > +		dma_iova_destroy(attachment->dev, state, priv->size, dir,
> > +				 attrs);
> 
> It is freed here too, but we can call map multiple times. Every time a
> move_notify happens can trigger another call to map.
> 
> I think just call unlink in those two and put dma_iova_free in detach

Yes, it can work.

Thanks

> 
> Jason