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Date:   Tue, 6 Aug 2019 07:31:11 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        kvm@vger.kernel.org, david@redhat.com, dave.hansen@intel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
        pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com,
        willy@infradead.org, lcapitulino@redhat.com, wei.w.wang@intel.com,
        aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com
Subject: Re: [PATCH v3 6/6] virtio-balloon: Add support for providing unused
 page reports to host
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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.

-- 
MST