Re: [PATCH v3 6/6] virtio-balloon: Add support for providing unused page reports to host

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 8/6/19 11:16 AM, Alexander Duyck wrote:
> On Tue, 2019-08-06 at 07:31 -0400, Michael S. Tsirkin wrote:
>> On Mon, Aug 05, 2019 at 09:27:16AM -0700, Alexander Duyck wrote:
>>> On Mon, 2019-08-05 at 12:00 -0400, Nitesh Narayan Lal wrote:
>>>> On 8/1/19 6:38 PM, Alexander Duyck wrote:
>>>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>>>
>>>>> Add support for the page reporting feature provided by virtio-balloon.
>>>>> Reporting differs from the regular balloon functionality in that is is
>>>>> much less durable than a standard memory balloon. Instead of creating a
>>>>> list of pages that cannot be accessed the pages are only inaccessible
>>>>> while they are being indicated to the virtio interface. Once the
>>>>> interface has acknowledged them they are placed back into their respective
>>>>> free lists and are once again accessible by the guest system.
>>>>>
>>>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>>>> ---
>>>>>  drivers/virtio/Kconfig              |    1 +
>>>>>  drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
>>>>>  include/uapi/linux/virtio_balloon.h |    1 +
>>>>>  3 files changed, 58 insertions(+)
>>>>>
>>>>> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
>>>>> index 078615cf2afc..4b2dd8259ff5 100644
>>>>> --- a/drivers/virtio/Kconfig
>>>>> +++ b/drivers/virtio/Kconfig
>>>>> @@ -58,6 +58,7 @@ config VIRTIO_BALLOON
>>>>>  	tristate "Virtio balloon driver"
>>>>>  	depends on VIRTIO
>>>>>  	select MEMORY_BALLOON
>>>>> +	select PAGE_REPORTING
>>>>>  	---help---
>>>>>  	 This driver supports increasing and decreasing the amount
>>>>>  	 of memory within a KVM guest.
>>>>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>>>>> index 2c19457ab573..971fe924e34f 100644
>>>>> --- a/drivers/virtio/virtio_balloon.c
>>>>> +++ b/drivers/virtio/virtio_balloon.c
>>>>> @@ -19,6 +19,7 @@
>>>>>  #include <linux/mount.h>
>>>>>  #include <linux/magic.h>
>>>>>  #include <linux/pseudo_fs.h>
>>>>> +#include <linux/page_reporting.h>
>>>>>  
>>>>>  /*
>>>>>   * Balloon device works in 4K page units.  So each page is pointed to by
>>>>> @@ -37,6 +38,9 @@
>>>>>  #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
>>>>>  	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
>>>>>  
>>>>> +/*  limit on the number of pages that can be on the reporting vq */
>>>>> +#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
>>>>> +
>>>>>  #ifdef CONFIG_BALLOON_COMPACTION
>>>>>  static struct vfsmount *balloon_mnt;
>>>>>  #endif
>>>>> @@ -46,6 +50,7 @@ enum virtio_balloon_vq {
>>>>>  	VIRTIO_BALLOON_VQ_DEFLATE,
>>>>>  	VIRTIO_BALLOON_VQ_STATS,
>>>>>  	VIRTIO_BALLOON_VQ_FREE_PAGE,
>>>>> +	VIRTIO_BALLOON_VQ_REPORTING,
>>>>>  	VIRTIO_BALLOON_VQ_MAX
>>>>>  };
>>>>>  
>>>>> @@ -113,6 +118,10 @@ struct virtio_balloon {
>>>>>  
>>>>>  	/* To register a shrinker to shrink memory upon memory pressure */
>>>>>  	struct shrinker shrinker;
>>>>> +
>>>>> +	/* Unused page reporting device */
>>>>> +	struct virtqueue *reporting_vq;
>>>>> +	struct page_reporting_dev_info ph_dev_info;
>>>>>  };
>>>>>  
>>>>>  static struct virtio_device_id id_table[] = {
>>>>> @@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
>>>>>  
>>>>>  }
>>>>>  
>>>>> +void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
>>>>> +				    unsigned int nents)
>>>>> +{
>>>>> +	struct virtio_balloon *vb =
>>>>> +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
>>>>> +	struct virtqueue *vq = vb->reporting_vq;
>>>>> +	unsigned int unused;
>>>>> +
>>>>> +	/* We should always be able to add these buffers to an empty queue. */
>>>>> +	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
>>>>> +			    GFP_NOWAIT | __GFP_NOWARN);
>>>> I think you should handle allocation failure here. It is a possibility, isn't?
>>>> Maybe return an error or even disable page hinting/reporting?
>>>>
>>> I don't think it is an issue I have to worry about. Specifically I am
>>> limiting the size of the scatterlist based on the size of the vq. As such
>>> I will never exceed the size and should be able to use it to store the
>>> scatterlist directly.
>> I agree. But it can't hurt to BUG_ON for good measure.
>>
> I wouldn't use a BUG_ON as that seems overkill. No need to panic the
> kernel just because we couldn't report some idle pages.
>
> I can probably do something like:
> 	if (WARN_ON(err))
> 		return;

+1

>
> That way the unused page reporting can run to completion still and the
> fact that we aren't really hinting on the pages would effectively be no
> different then if we had a direct assigned device or shared memory in the
> hypervisor.
>