Re: [PATCH] xen/gntdev: remove struct gntdev_copy_batch from stack

["Jürgen Groß" <jgross@suse.com>]

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On 11.07.25 03:03, Stefano Stabellini wrote:
> On Wed, 9 Jul 2025, Jürgen Groß wrote:
>> On 08.07.25 21:01, Stefano Stabellini wrote:
>>> On Thu, 3 Jul 2025, Juergen Gross wrote:
>>>> When compiling the kernel with LLVM, the following warning was issued:
>>>>
>>>>     drivers/xen/gntdev.c:991: warning: stack frame size (1160) exceeds
>>>>     limit (1024) in function 'gntdev_ioctl'
>>>>
>>>> The main reason is struct gntdev_copy_batch which is located on the
>>>> stack and has a size of nearly 1kb.
>>>>
>>>> For performance reasons it shouldn't by just dynamically allocated
>>>> instead, so allocate a new instance when needed and instead of freeing
>>>> it put it into a list of free structs anchored in struct gntdev_priv.
>>>>
>>>> Fixes: a4cdb556cae0 ("xen/gntdev: add ioctl for grant copy")
>>>> Reported-by: Abinash Singh <abinashsinghlalotra@gmail.com>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>> ---
>>>>    drivers/xen/gntdev-common.h |  4 +++
>>>>    drivers/xen/gntdev.c        | 71 ++++++++++++++++++++++++++-----------
>>>>    2 files changed, 54 insertions(+), 21 deletions(-)
>>>>
>>>> diff --git a/drivers/xen/gntdev-common.h b/drivers/xen/gntdev-common.h
>>>> index 9c286b2a1900..ac8ce3179ba2 100644
>>>> --- a/drivers/xen/gntdev-common.h
>>>> +++ b/drivers/xen/gntdev-common.h
>>>> @@ -26,6 +26,10 @@ struct gntdev_priv {
>>>>    	/* lock protects maps and freeable_maps. */
>>>>    	struct mutex lock;
>>>>    +	/* Free instances of struct gntdev_copy_batch. */
>>>> +	struct gntdev_copy_batch *batch;
>>>> +	struct mutex batch_lock;
>>>> +
>>>>    #ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>>>    	/* Device for which DMA memory is allocated. */
>>>>    	struct device *dma_dev;
>>>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>>>> index 61faea1f0663..1f2160765618 100644
>>>> --- a/drivers/xen/gntdev.c
>>>> +++ b/drivers/xen/gntdev.c
>>>> @@ -56,6 +56,18 @@ MODULE_AUTHOR("Derek G. Murray
>>>> <Derek.Murray@cl.cam.ac.uk>, "
>>>>    	      "Gerd Hoffmann <kraxel@redhat.com>");
>>>>    MODULE_DESCRIPTION("User-space granted page access driver");
>>>>    +#define GNTDEV_COPY_BATCH 16
>>>> +
>>>> +struct gntdev_copy_batch {
>>>> +	struct gnttab_copy ops[GNTDEV_COPY_BATCH];
>>>> +	struct page *pages[GNTDEV_COPY_BATCH];
>>>> +	s16 __user *status[GNTDEV_COPY_BATCH];
>>>> +	unsigned int nr_ops;
>>>> +	unsigned int nr_pages;
>>>> +	bool writeable;
>>>> +	struct gntdev_copy_batch *next;
>>>> +};
>>>> +
>>>>    static unsigned int limit = 64*1024;
>>>>    module_param(limit, uint, 0644);
>>>>    MODULE_PARM_DESC(limit,
>>>> @@ -584,6 +596,8 @@ static int gntdev_open(struct inode *inode, struct
>>>> file *flip)
>>>>    	INIT_LIST_HEAD(&priv->maps);
>>>>    	mutex_init(&priv->lock);
>>>>    +	mutex_init(&priv->batch_lock);
>>>> +
>>>>    #ifdef CONFIG_XEN_GNTDEV_DMABUF
>>>>    	priv->dmabuf_priv = gntdev_dmabuf_init(flip);
>>>>    	if (IS_ERR(priv->dmabuf_priv)) {
>>>> @@ -608,6 +622,7 @@ static int gntdev_release(struct inode *inode, struct
>>>> file *flip)
>>>>    {
>>>>    	struct gntdev_priv *priv = flip->private_data;
>>>>    	struct gntdev_grant_map *map;
>>>> +	struct gntdev_copy_batch *batch;
>>>>      	pr_debug("priv %p\n", priv);
>>>>    @@ -620,6 +635,14 @@ static int gntdev_release(struct inode *inode,
>>>> struct file *flip)
>>>>    	}
>>>>    	mutex_unlock(&priv->lock);
>>>>    +	mutex_lock(&priv->batch_lock);
>>>> +	while (priv->batch) {
>>>> +		batch = priv->batch;
>>>> +		priv->batch = batch->next;
>>>> +		kfree(batch);
>>>> +	}
>>>> +	mutex_unlock(&priv->batch_lock);
>>>> +
>>>>    #ifdef CONFIG_XEN_GNTDEV_DMABUF
>>>>    	gntdev_dmabuf_fini(priv->dmabuf_priv);
>>>>    #endif
>>>> @@ -785,17 +808,6 @@ static long gntdev_ioctl_notify(struct gntdev_priv
>>>> *priv, void __user *u)
>>>>    	return rc;
>>>>    }
>>>>    -#define GNTDEV_COPY_BATCH 16
>>>> -
>>>> -struct gntdev_copy_batch {
>>>> -	struct gnttab_copy ops[GNTDEV_COPY_BATCH];
>>>> -	struct page *pages[GNTDEV_COPY_BATCH];
>>>> -	s16 __user *status[GNTDEV_COPY_BATCH];
>>>> -	unsigned int nr_ops;
>>>> -	unsigned int nr_pages;
>>>> -	bool writeable;
>>>> -};
>>>> -
>>>>    static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user
>>>> *virt,
>>>>    				unsigned long *gfn)
>>>>    {
>>>> @@ -953,36 +965,53 @@ static int gntdev_grant_copy_seg(struct
>>>> gntdev_copy_batch *batch,
>>>>    static long gntdev_ioctl_grant_copy(struct gntdev_priv *priv, void
>>>> __user *u)
>>>>    {
>>>>    	struct ioctl_gntdev_grant_copy copy;
>>>> -	struct gntdev_copy_batch batch;
>>>> +	struct gntdev_copy_batch *batch;
>>>>    	unsigned int i;
>>>>    	int ret = 0;
>>>>      	if (copy_from_user(&copy, u, sizeof(copy)))
>>>>    		return -EFAULT;
>>>>    -	batch.nr_ops = 0;
>>>> -	batch.nr_pages = 0;
>>>> +	mutex_lock(&priv->batch_lock);
>>>> +	if (!priv->batch) {
>>>> +		batch = kmalloc(sizeof(*batch), GFP_KERNEL);
>>>> +	} else {
>>>> +		batch = priv->batch;
>>>> +		priv->batch = batch->next;
>>>> +	}
>>>> +	mutex_unlock(&priv->batch_lock);
>>>
>>> I am concerned about the potentially unbounded amount of memory that
>>> could be allocated this way.
>>
>> Unbounded? It can be at most the number of threads using the interface
>> concurrently.
> 
> That's what I meant

1 kB additional memory per thread won't be the end of the world.
Each thread will consume much more memory for other purposes anyway.

>>> The mutex is already a potentially very slow operation. Could we instead
>>> allocate a single batch, and if it is currently in use, use the mutex to
>>> wait until it becomes available?
>>
>> As this interface is e.g. used by the qemu based qdisk backend, the chances
>> are very high that there are concurrent users. This would hurt multi-ring
>> qdisk quite badly!
>>
>> It would be possible to replace the mutex with a spinlock and do the kmalloc()
>> outside the locked region.
>>
>>>
>>> I am also OK with the current approach but I thought I would ask.
>>>
>>>
>>>
>>>
>>>> +	if (!batch)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	batch->nr_ops = 0;
>>>> +	batch->nr_pages = 0;
>>>>      	for (i = 0; i < copy.count; i++) {
>>>>    		struct gntdev_grant_copy_segment seg;
>>>>      		if (copy_from_user(&seg, &copy.segments[i],
>>>> sizeof(seg))) {
>>>>    			ret = -EFAULT;
>>>> +			gntdev_put_pages(batch);
>>>>    			goto out;
>>>>    		}
>>>>    -		ret = gntdev_grant_copy_seg(&batch, &seg,
>>>> &copy.segments[i].status);
>>>> -		if (ret < 0)
>>>> +		ret = gntdev_grant_copy_seg(batch, &seg,
>>>> &copy.segments[i].status);
>>>> +		if (ret < 0) {
>>>> +			gntdev_put_pages(batch);
>>>>    			goto out;
>>>> +		}
>>>>      		cond_resched();
>>>>    	}
>>>> -	if (batch.nr_ops)
>>>> -		ret = gntdev_copy(&batch);
>>>> -	return ret;
>>>> +	if (batch->nr_ops)
>>>> +		ret = gntdev_copy(batch);
>>>> +
>>>> + out:
>>>> +	mutex_lock(&priv->batch_lock);
>>>> +	batch->next = priv->batch;
>>>> +	priv->batch = batch;
>>>> +	mutex_unlock(&priv->batch_lock);
>>>>    -  out:
>>>> -	gntdev_put_pages(&batch);
>>>
>>> One change from before is that in case of no errors, gntdev_put_pages is
>>> not called anymore. Do we want that? Specifically, we are missing the
>>> call to unpin_user_pages_dirty_lock
>>
>> I don't think you are right. There was a "return ret" before the "out:"
>> label before my patch.
> 
> You are right, I missed it.
> 
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

Thanks,


Juergen

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