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Subject: Re: [PATCH 6/8] xen/gntdev: Implement dma-buf export functionality
To:     Dongwon Kim <dongwon.kim@intel.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        jgross@suse.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, daniel.vetter@intel.com,
        matthew.d.roper@intel.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <20180525153331.31188-1-andr2000@gmail.com>
 <20180525153331.31188-7-andr2000@gmail.com>
 <20180530231006.GA2929@downor-Z87X-UD5H>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <072fb651-52db-05bd-d110-ada904bcac3d@gmail.com>
Date:   Thu, 31 May 2018 08:55:27 +0300
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On 05/31/2018 02:10 AM, Dongwon Kim wrote:
> On Fri, May 25, 2018 at 06:33:29PM +0300, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>
>> 1. Create a dma-buf from grant references provided by the foreign
>>     domain. By default dma-buf is backed by system memory pages, but
>>     by providing GNTDEV_DMA_FLAG_XXX flags it can also be created
>>     as a DMA write-combine/coherent buffer, e.g. allocated with
>>     corresponding dma_alloc_xxx API.
>>     Export the resulting buffer as a new dma-buf.
>>
>> 2. Implement waiting for the dma-buf to be released: block until the
>>     dma-buf with the file descriptor provided is released.
>>     If within the time-out provided the buffer is not released then
>>     -ETIMEDOUT error is returned. If the buffer with the file descriptor
>>     does not exist or has already been released, then -ENOENT is returned.
>>     For valid file descriptors this must not be treated as error.
>>
>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>> ---
>>   drivers/xen/gntdev.c | 478 ++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 476 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>> index 9e450622af1a..52abc6cd5846 100644
>> --- a/drivers/xen/gntdev.c
>> +++ b/drivers/xen/gntdev.c
>> @@ -4,6 +4,8 @@
>>    * Device for accessing (in user-space) pages that have been granted by other
>>    * domains.
>>    *
>> + * DMA buffer implementation is based on drivers/gpu/drm/drm_prime.c.
>> + *
>>    * Copyright (c) 2006-2007, D G Murray.
>>    *           (c) 2009 Gerd Hoffmann <kraxel@redhat.com>
>>    *           (c) 2018 Oleksandr Andrushchenko, EPAM Systems Inc.
>> @@ -41,6 +43,9 @@
>>   #ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>   #include <linux/of_device.h>
>>   #endif
>> +#ifdef CONFIG_XEN_GNTDEV_DMABUF
>> +#include <linux/dma-buf.h>
>> +#endif
>>   
>>   #include <xen/xen.h>
>>   #include <xen/grant_table.h>
>> @@ -81,6 +86,17 @@ struct gntdev_priv {
>>   	/* Device for which DMA memory is allocated. */
>>   	struct device *dma_dev;
>>   #endif
>> +
>> +#ifdef CONFIG_XEN_GNTDEV_DMABUF
>> +	/* Private data of the hyper DMA buffers. */
>> +
>> +	/* List of exported DMA buffers. */
>> +	struct list_head dmabuf_exp_list;
>> +	/* List of wait objects. */
>> +	struct list_head dmabuf_exp_wait_list;
>> +	/* This is the lock which protects dma_buf_xxx lists. */
>> +	struct mutex dmabuf_lock;
>> +#endif
>>   };
>>   
>>   struct unmap_notify {
>> @@ -125,12 +141,38 @@ struct grant_map {
>>   
>>   #ifdef CONFIG_XEN_GNTDEV_DMABUF
>>   struct xen_dmabuf {
>> +	struct gntdev_priv *priv;
>> +	struct dma_buf *dmabuf;
>> +	struct list_head next;
>> +	int fd;
>> +
>>   	union {
>> +		struct {
>> +			/* Exported buffers are reference counted. */
>> +			struct kref refcount;
>> +			struct grant_map *map;
>> +		} exp;
>>   		struct {
>>   			/* Granted references of the imported buffer. */
>>   			grant_ref_t *refs;
>>   		} imp;
>>   	} u;
>> +
>> +	/* Number of pages this buffer has. */
>> +	int nr_pages;
>> +	/* Pages of this buffer. */
>> +	struct page **pages;
>> +};
>> +
>> +struct xen_dmabuf_wait_obj {
>> +	struct list_head next;
>> +	struct xen_dmabuf *xen_dmabuf;
>> +	struct completion completion;
>> +};
>> +
>> +struct xen_dmabuf_attachment {
>> +	struct sg_table *sgt;
>> +	enum dma_data_direction dir;
>>   };
>>   #endif
>>   
>> @@ -320,6 +362,16 @@ static void gntdev_put_map(struct gntdev_priv *priv, struct grant_map *map)
>>   	gntdev_free_map(map);
>>   }
>>   
>> +#ifdef CONFIG_XEN_GNTDEV_DMABUF
>> +static void gntdev_remove_map(struct gntdev_priv *priv, struct grant_map *map)
>> +{
>> +	mutex_lock(&priv->lock);
>> +	list_del(&map->next);
>> +	gntdev_put_map(NULL /* already removed */, map);
>> +	mutex_unlock(&priv->lock);
>> +}
>> +#endif
>> +
>>   /* ------------------------------------------------------------------ */
>>   
>>   static int find_grant_ptes(pte_t *pte, pgtable_t token,
>> @@ -628,6 +680,12 @@ static int gntdev_open(struct inode *inode, struct file *flip)
>>   	INIT_LIST_HEAD(&priv->freeable_maps);
>>   	mutex_init(&priv->lock);
>>   
>> +#ifdef CONFIG_XEN_GNTDEV_DMABUF
>> +	mutex_init(&priv->dmabuf_lock);
>> +	INIT_LIST_HEAD(&priv->dmabuf_exp_list);
>> +	INIT_LIST_HEAD(&priv->dmabuf_exp_wait_list);
>> +#endif
>> +
>>   	if (use_ptemod) {
>>   		priv->mm = get_task_mm(current);
>>   		if (!priv->mm) {
>> @@ -1053,17 +1111,433 @@ static long gntdev_ioctl_grant_copy(struct gntdev_priv *priv, void __user *u)
>>   /* DMA buffer export support.                                         */
>>   /* ------------------------------------------------------------------ */
>>   
>> +/* ------------------------------------------------------------------ */
>> +/* Implementation of wait for exported DMA buffer to be released.     */
>> +/* ------------------------------------------------------------------ */
>> +
>> +static void dmabuf_exp_release(struct kref *kref);
>> +
>> +static struct xen_dmabuf_wait_obj *
>> +dmabuf_exp_wait_obj_new(struct gntdev_priv *priv,
>> +			struct xen_dmabuf *xen_dmabuf)
>> +{
>> +	struct xen_dmabuf_wait_obj *obj;
>> +
>> +	obj = kzalloc(sizeof(*obj), GFP_KERNEL);
>> +	if (!obj)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	init_completion(&obj->completion);
>> +	obj->xen_dmabuf = xen_dmabuf;
>> +
>> +	mutex_lock(&priv->dmabuf_lock);
>> +	list_add(&obj->next, &priv->dmabuf_exp_wait_list);
>> +	/* Put our reference and wait for xen_dmabuf's release to fire. */
>> +	kref_put(&xen_dmabuf->u.exp.refcount, dmabuf_exp_release);
>> +	mutex_unlock(&priv->dmabuf_lock);
>> +	return obj;
>> +}
>> +
>> +static void dmabuf_exp_wait_obj_free(struct gntdev_priv *priv,
>> +				     struct xen_dmabuf_wait_obj *obj)
>> +{
>> +	struct xen_dmabuf_wait_obj *cur_obj, *q;
>> +
>> +	mutex_lock(&priv->dmabuf_lock);
>> +	list_for_each_entry_safe(cur_obj, q, &priv->dmabuf_exp_wait_list, next)
>> +		if (cur_obj == obj) {
>> +			list_del(&obj->next);
>> +			kfree(obj);
>> +			break;
>> +		}
>> +	mutex_unlock(&priv->dmabuf_lock);
>> +}
>> +
>> +static int dmabuf_exp_wait_obj_wait(struct xen_dmabuf_wait_obj *obj,
>> +				    u32 wait_to_ms)
>> +{
>> +	if (wait_for_completion_timeout(&obj->completion,
>> +			msecs_to_jiffies(wait_to_ms)) <= 0)
>> +		return -ETIMEDOUT;
>> +
>> +	return 0;
>> +}
>> +
>> +static void dmabuf_exp_wait_obj_signal(struct gntdev_priv *priv,
>> +				       struct xen_dmabuf *xen_dmabuf)
>> +{
>> +	struct xen_dmabuf_wait_obj *obj, *q;
>> +
>> +	list_for_each_entry_safe(obj, q, &priv->dmabuf_exp_wait_list, next)
>> +		if (obj->xen_dmabuf == xen_dmabuf) {
>> +			pr_debug("Found xen_dmabuf in the wait list, wake\n");
>> +			complete_all(&obj->completion);
>> +		}
>> +}
>> +
>> +static struct xen_dmabuf *
>> +dmabuf_exp_wait_obj_get_by_fd(struct gntdev_priv *priv, int fd)
>> +{
>> +	struct xen_dmabuf *q, *xen_dmabuf, *ret = ERR_PTR(-ENOENT);
>> +
>> +	mutex_lock(&priv->dmabuf_lock);
>> +	list_for_each_entry_safe(xen_dmabuf, q, &priv->dmabuf_exp_list, next)
>> +		if (xen_dmabuf->fd == fd) {
>> +			pr_debug("Found xen_dmabuf in the wait list\n");
>> +			kref_get(&xen_dmabuf->u.exp.refcount);
>> +			ret = xen_dmabuf;
>> +			break;
>> +		}
>> +	mutex_unlock(&priv->dmabuf_lock);
>> +	return ret;
>> +}
>> +
>>   static int dmabuf_exp_wait_released(struct gntdev_priv *priv, int fd,
>>   				    int wait_to_ms)
>>   {
>> -	return -ETIMEDOUT;
>> +	struct xen_dmabuf *xen_dmabuf;
>> +	struct xen_dmabuf_wait_obj *obj;
>> +	int ret;
>> +
>> +	pr_debug("Will wait for dma-buf with fd %d\n", fd);
>> +	/*
>> +	 * Try to find the DMA buffer: if not found means that
>> +	 * either the buffer has already been released or file descriptor
>> +	 * provided is wrong.
>> +	 */
>> +	xen_dmabuf = dmabuf_exp_wait_obj_get_by_fd(priv, fd);
>> +	if (IS_ERR(xen_dmabuf))
>> +		return PTR_ERR(xen_dmabuf);
>> +
>> +	/*
>> +	 * xen_dmabuf still exists and is reference count locked by us now,
>> +	 * so prepare to wait: allocate wait object and add it to the wait list,
>> +	 * so we can find it on release.
>> +	 */
>> +	obj = dmabuf_exp_wait_obj_new(priv, xen_dmabuf);
>> +	if (IS_ERR(obj)) {
>> +		pr_err("Failed to setup wait object, ret %ld\n", PTR_ERR(obj));
>> +		return PTR_ERR(obj);
>> +	}
>> +
>> +	ret = dmabuf_exp_wait_obj_wait(obj, wait_to_ms);
>> +	dmabuf_exp_wait_obj_free(priv, obj);
>> +	return ret;
>> +}
>> +
>> +/* ------------------------------------------------------------------ */
>> +/* DMA buffer export support.                                         */
>> +/* ------------------------------------------------------------------ */
>> +
>> +static struct sg_table *
>> +dmabuf_pages_to_sgt(struct page **pages, unsigned int nr_pages)
>> +{
>> +	struct sg_table *sgt;
>> +	int ret;
>> +
>> +	sgt = kmalloc(sizeof(*sgt), GFP_KERNEL);
>> +	if (!sgt) {
>> +		ret = -ENOMEM;
>> +		goto out;
>> +	}
>> +
>> +	ret = sg_alloc_table_from_pages(sgt, pages, nr_pages, 0,
>> +					nr_pages << PAGE_SHIFT,
>> +					GFP_KERNEL);
>> +	if (ret)
>> +		goto out;
>> +
>> +	return sgt;
>> +
>> +out:
>> +	kfree(sgt);
>> +	return ERR_PTR(ret);
>> +}
>> +
>> +static int dmabuf_exp_ops_attach(struct dma_buf *dma_buf,
>> +				 struct device *target_dev,
>> +				 struct dma_buf_attachment *attach)
>> +{
>> +	struct xen_dmabuf_attachment *xen_dmabuf_attach;
>> +
>> +	xen_dmabuf_attach = kzalloc(sizeof(*xen_dmabuf_attach), GFP_KERNEL);
>> +	if (!xen_dmabuf_attach)
>> +		return -ENOMEM;
>> +
>> +	xen_dmabuf_attach->dir = DMA_NONE;
>> +	attach->priv = xen_dmabuf_attach;
>> +	/* Might need to pin the pages of the buffer now. */
>> +	return 0;
>> +}
>> +
>> +static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,
>> +				  struct dma_buf_attachment *attach)
>> +{
>> +	struct xen_dmabuf_attachment *xen_dmabuf_attach = attach->priv;
>> +
>> +	if (xen_dmabuf_attach) {
>> +		struct sg_table *sgt = xen_dmabuf_attach->sgt;
>> +
>> +		if (sgt) {
>> +			if (xen_dmabuf_attach->dir != DMA_NONE)
>> +				dma_unmap_sg_attrs(attach->dev, sgt->sgl,
>> +						   sgt->nents,
>> +						   xen_dmabuf_attach->dir,
>> +						   DMA_ATTR_SKIP_CPU_SYNC);
>> +			sg_free_table(sgt);
>> +		}
>> +
>> +		kfree(sgt);
>> +		kfree(xen_dmabuf_attach);
>> +		attach->priv = NULL;
>> +	}
>> +	/* Might need to unpin the pages of the buffer now. */
>> +}
>> +
>> +static struct sg_table *
>> +dmabuf_exp_ops_map_dma_buf(struct dma_buf_attachment *attach,
>> +			   enum dma_data_direction dir)
>> +{
>> +	struct xen_dmabuf_attachment *xen_dmabuf_attach = attach->priv;
>> +	struct xen_dmabuf *xen_dmabuf = attach->dmabuf->priv;
>> +	struct sg_table *sgt;
>> +
>> +	pr_debug("Mapping %d pages for dev %p\n", xen_dmabuf->nr_pages,
>> +		 attach->dev);
>> +
>> +	if (WARN_ON(dir == DMA_NONE || !xen_dmabuf_attach))
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	/* Return the cached mapping when possible. */
>> +	if (xen_dmabuf_attach->dir == dir)
>> +		return xen_dmabuf_attach->sgt;
> may need to check xen_dmabuf_attach->sgt == NULL (i.e. first time mapping)?
> Also, I am not sure if this mechanism of reusing previously generated sgt
> for other mappings is universally ok for any use-cases... I don't know if
> it is acceptable as per the specification.
Well, I was not sure about this piece of code as well,
so I'll probably allocate a new sgt each time and do not reuse it
as now
>> +
>> +	/*
>> +	 * Two mappings with different directions for the same attachment are
>> +	 * not allowed.
>> +	 */
>> +	if (WARN_ON(xen_dmabuf_attach->dir != DMA_NONE))
>> +		return ERR_PTR(-EBUSY);
>> +
>> +	sgt = dmabuf_pages_to_sgt(xen_dmabuf->pages, xen_dmabuf->nr_pages);
>> +	if (!IS_ERR(sgt)) {
>> +		if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
>> +				      DMA_ATTR_SKIP_CPU_SYNC)) {
>> +			sg_free_table(sgt);
>> +			kfree(sgt);
>> +			sgt = ERR_PTR(-ENOMEM);
>> +		} else {
>> +			xen_dmabuf_attach->sgt = sgt;
>> +			xen_dmabuf_attach->dir = dir;
>> +		}
>> +	}
>> +	if (IS_ERR(sgt))
>> +		pr_err("Failed to map sg table for dev %p\n", attach->dev);
>> +	return sgt;
>> +}
>> +
>> +static void dmabuf_exp_ops_unmap_dma_buf(struct dma_buf_attachment *attach,
>> +					 struct sg_table *sgt,
>> +					 enum dma_data_direction dir)
>> +{
>> +	/* Not implemented. The unmap is done at dmabuf_exp_ops_detach(). */
> Not sure if it's ok to do nothing here because the spec says this function is
> mandatory and it should unmap and "release" &sg_table associated with it.
>
> 	/**
> 	 * @unmap_dma_buf:
> 	 *
> 	 * This is called by dma_buf_unmap_attachment() and should unmap and
> 	 * release the &sg_table allocated in @map_dma_buf, and it is mandatory.
> 	 * It should also unpin the backing storage if this is the last mapping
> 	 * of the DMA buffer, it the exporter supports backing storage
> 	 * migration.
> 	 */
Yes, as I say at the top of the file dma-buf handling is DRM PRIME
based, so I have the workflow just like in there.
Do you think we have to be more strict and rework this?

Daniel, what do you think?
>> +}
>> +
>> +static void dmabuf_exp_release(struct kref *kref)
>> +{
>> +	struct xen_dmabuf *xen_dmabuf =
>> +		container_of(kref, struct xen_dmabuf, u.exp.refcount);
>> +
>> +	dmabuf_exp_wait_obj_signal(xen_dmabuf->priv, xen_dmabuf);
>> +	list_del(&xen_dmabuf->next);
>> +	kfree(xen_dmabuf);
>> +}
>> +
>> +static void dmabuf_exp_ops_release(struct dma_buf *dma_buf)
>> +{
>> +	struct xen_dmabuf *xen_dmabuf = dma_buf->priv;
>> +	struct gntdev_priv *priv = xen_dmabuf->priv;
>> +
>> +	gntdev_remove_map(priv, xen_dmabuf->u.exp.map);
>> +	mutex_lock(&priv->dmabuf_lock);
>> +	kref_put(&xen_dmabuf->u.exp.refcount, dmabuf_exp_release);
>> +	mutex_unlock(&priv->dmabuf_lock);
>> +}
>> +
>> +static void *dmabuf_exp_ops_kmap_atomic(struct dma_buf *dma_buf,
>> +					unsigned long page_num)
>> +{
>> +	/* Not implemented. */
>> +	return NULL;
>> +}
>> +
>> +static void dmabuf_exp_ops_kunmap_atomic(struct dma_buf *dma_buf,
>> +					 unsigned long page_num, void *addr)
>> +{
>> +	/* Not implemented. */
>> +}
>> +
>> +static void *dmabuf_exp_ops_kmap(struct dma_buf *dma_buf,
>> +				 unsigned long page_num)
>> +{
>> +	/* Not implemented. */
>> +	return NULL;
>> +}
>> +
>> +static void dmabuf_exp_ops_kunmap(struct dma_buf *dma_buf,
>> +				  unsigned long page_num, void *addr)
>> +{
>> +	/* Not implemented. */
>> +}
>> +
>> +static int dmabuf_exp_ops_mmap(struct dma_buf *dma_buf,
>> +			       struct vm_area_struct *vma)
>> +{
>> +	/* Not implemented. */
>> +	return 0;
>> +}
>> +
>> +static const struct dma_buf_ops dmabuf_exp_ops =  {
>> +	.attach = dmabuf_exp_ops_attach,
>> +	.detach = dmabuf_exp_ops_detach,
>> +	.map_dma_buf = dmabuf_exp_ops_map_dma_buf,
>> +	.unmap_dma_buf = dmabuf_exp_ops_unmap_dma_buf,
>> +	.release = dmabuf_exp_ops_release,
>> +	.map = dmabuf_exp_ops_kmap,
>> +	.map_atomic = dmabuf_exp_ops_kmap_atomic,
>> +	.unmap = dmabuf_exp_ops_kunmap,
>> +	.unmap_atomic = dmabuf_exp_ops_kunmap_atomic,
>> +	.mmap = dmabuf_exp_ops_mmap,
>> +};
>> +
>> +static int dmabuf_export(struct gntdev_priv *priv, struct grant_map *map,
>> +			 int *fd)
>> +{
>> +	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
>> +	struct xen_dmabuf *xen_dmabuf;
>> +	int ret = 0;
>> +
>> +	xen_dmabuf = kzalloc(sizeof(*xen_dmabuf), GFP_KERNEL);
>> +	if (!xen_dmabuf)
>> +		return -ENOMEM;
>> +
>> +	kref_init(&xen_dmabuf->u.exp.refcount);
>> +
>> +	xen_dmabuf->priv = priv;
>> +	xen_dmabuf->nr_pages = map->count;
>> +	xen_dmabuf->pages = map->pages;
>> +	xen_dmabuf->u.exp.map = map;
>> +
>> +	exp_info.exp_name = KBUILD_MODNAME;
>> +	if (map->dma_dev->driver && map->dma_dev->driver->owner)
>> +		exp_info.owner = map->dma_dev->driver->owner;
>> +	else
>> +		exp_info.owner = THIS_MODULE;
>> +	exp_info.ops = &dmabuf_exp_ops;
>> +	exp_info.size = map->count << PAGE_SHIFT;
>> +	exp_info.flags = O_RDWR;
>> +	exp_info.priv = xen_dmabuf;
>> +
>> +	xen_dmabuf->dmabuf = dma_buf_export(&exp_info);
>> +	if (IS_ERR(xen_dmabuf->dmabuf)) {
>> +		ret = PTR_ERR(xen_dmabuf->dmabuf);
>> +		xen_dmabuf->dmabuf = NULL;
>> +		goto fail;
>> +	}
>> +
>> +	ret = dma_buf_fd(xen_dmabuf->dmabuf, O_CLOEXEC);
>> +	if (ret < 0)
>> +		goto fail;
>> +
>> +	xen_dmabuf->fd = ret;
>> +	*fd = ret;
>> +
>> +	pr_debug("Exporting DMA buffer with fd %d\n", ret);
>> +
>> +	mutex_lock(&priv->dmabuf_lock);
>> +	list_add(&xen_dmabuf->next, &priv->dmabuf_exp_list);
>> +	mutex_unlock(&priv->dmabuf_lock);
>> +	return 0;
>> +
>> +fail:
>> +	if (xen_dmabuf->dmabuf)
>> +		dma_buf_put(xen_dmabuf->dmabuf);
>> +	kfree(xen_dmabuf);
>> +	return ret;
>> +}
>> +
>> +static struct grant_map *
>> +dmabuf_exp_alloc_backing_storage(struct gntdev_priv *priv, int dmabuf_flags,
>> +				 int count)
>> +{
>> +	struct grant_map *map;
>> +
>> +	if (unlikely(count <= 0))
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	if ((dmabuf_flags & GNTDEV_DMA_FLAG_WC) &&
>> +	    (dmabuf_flags & GNTDEV_DMA_FLAG_COHERENT)) {
>> +		pr_err("Wrong dma-buf flags: either WC or coherent, not both\n");
>> +		return ERR_PTR(-EINVAL);
>> +	}
>> +
>> +	map = gntdev_alloc_map(priv, count, dmabuf_flags);
>> +	if (!map)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	if (unlikely(atomic_add_return(count, &pages_mapped) > limit)) {
>> +		pr_err("can't map: over limit\n");
>> +		gntdev_put_map(NULL, map);
>> +		return ERR_PTR(-ENOMEM);
>> +	}
>> +	return map;
>>   }
> When and how would this allocation be freed? I don't see any ioctl for freeing up
> shared pages.
on xen_dmabuf.release callback which is refcounted
>>   
>>   static int dmabuf_exp_from_refs(struct gntdev_priv *priv, int flags,
>>   				int count, u32 domid, u32 *refs, u32 *fd)
>>   {
>> +	struct grant_map *map;
>> +	int i, ret;
>> +
>>   	*fd = -1;
>> -	return -EINVAL;
>> +
>> +	if (use_ptemod) {
>> +		pr_err("Cannot provide dma-buf: use_ptemode %d\n",
>> +		       use_ptemod);
>> +		return -EINVAL;
>> +	}
>> +
>> +	map = dmabuf_exp_alloc_backing_storage(priv, flags, count);
>> +	if (IS_ERR(map))
>> +		return PTR_ERR(map);
>> +
>> +	for (i = 0; i < count; i++) {
>> +		map->grants[i].domid = domid;
>> +		map->grants[i].ref = refs[i];
>> +	}
>> +
>> +	mutex_lock(&priv->lock);
>> +	gntdev_add_map(priv, map);
>> +	mutex_unlock(&priv->lock);
>> +
>> +	map->flags |= GNTMAP_host_map;
>> +#if defined(CONFIG_X86)
>> +	map->flags |= GNTMAP_device_map;
>> +#endif
>> +
>> +	ret = map_grant_pages(map);
>> +	if (ret < 0)
>> +		goto out;
>> +
>> +	ret = dmabuf_export(priv, map, fd);
>> +	if (ret < 0)
>> +		goto out;
>> +
>> +	return 0;
>> +
>> +out:
>> +	gntdev_remove_map(priv, map);
>> +	return ret;
>>   }
>>   
>>   /* ------------------------------------------------------------------ */
>> -- 
>> 2.17.0
>>