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Subject: Re: [PATCH v2 5/5] virtio-balloon: Add support for providing page
 hints to host
From:   Alexander Duyck <alexander.h.duyck@linux.intel.com>
To:     Nitesh Narayan Lal <nitesh@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     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,
        lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com,
        pbonzini@redhat.com, dan.j.williams@intel.com
Date:   Thu, 25 Jul 2019 07:56:15 -0700
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On Thu, 2019-07-25 at 10:44 -0400, Nitesh Narayan Lal wrote:
> On 7/24/19 3:02 PM, Michael S. Tsirkin wrote:
> > On Wed, Jul 24, 2019 at 10:05:14AM -0700, Alexander Duyck wrote:
> > > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > > 
> > > Add support for the page hinting feature provided by virtio-balloon.
> > > Hinting 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>
> > Looking at the design, it seems that hinted pages can immediately be
> > reused. I wonder how we can efficiently support this
> > with kvm when poisoning is in effect. Of course we can just
> > ignore the poison. However it seems cleaner to
> > 1. verify page is poisoned with the correct value
> > 2. fill the page with the correct value on fault
> > 
> > Requirement 2 requires some kind of madvise that
> > will save the poison e.g. in the VMA.
> > 
> > Not a blocker for sure ... 
> > 
> > 
> > > ---
> > >  drivers/virtio/Kconfig              |    1 +
> > >  drivers/virtio/virtio_balloon.c     |   47 +++++++++++++++++++++++++++++++++++
> > >  include/uapi/linux/virtio_balloon.h |    1 +
> > >  3 files changed, 49 insertions(+)
> > > 
> > > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> > > index 078615cf2afc..d45556ae1f81 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_HINTING
> > >  	---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 226fbb995fb0..dee9f8f3ad09 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_hinting.h>
> > >  
> > >  /*
> > >   * Balloon device works in 4K page units.  So each page is pointed to by
> > > @@ -27,6 +28,7 @@
> > >   */
> > >  #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
> > >  #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> > > +#define VIRTIO_BALLOON_ARRAY_HINTS_MAX	32
> > >  #define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> > >  
> > >  #define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \
> > > @@ -46,6 +48,7 @@ enum virtio_balloon_vq {
> > >  	VIRTIO_BALLOON_VQ_DEFLATE,
> > >  	VIRTIO_BALLOON_VQ_STATS,
> > >  	VIRTIO_BALLOON_VQ_FREE_PAGE,
> > > +	VIRTIO_BALLOON_VQ_HINTING,
> > >  	VIRTIO_BALLOON_VQ_MAX
> > >  };
> > >  
> > > @@ -113,6 +116,10 @@ struct virtio_balloon {
> > >  
> > >  	/* To register a shrinker to shrink memory upon memory pressure */
> > >  	struct shrinker shrinker;
> > > +
> > > +	/* Unused page hinting device */
> > > +	struct virtqueue *hinting_vq;
> > > +	struct page_hinting_dev_info ph_dev_info;
> > >  };
> > >  
> > >  static struct virtio_device_id id_table[] = {
> > > @@ -152,6 +159,22 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> > >  
> > >  }
> > >  
> > > +void virtballoon_page_hinting_react(struct page_hinting_dev_info *ph_dev_info,
> > > +				    unsigned int num_hints)
> > > +{
> > > +	struct virtio_balloon *vb =
> > > +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
> > > +	struct virtqueue *vq = vb->hinting_vq;
> > > +	unsigned int unused;
> > > +
> > > +	/* We should always be able to add these buffers to an empty queue. */
> > 
> > can be an out of memory condition, and then ...
> 
> Do we need an error check here?
> 
> For situations where this fails we should disable hinting completely, maybe?

No. Instead I will just limit the capacity to no more than the vq size.
Doing that should allow us to avoid the out of memory issue here if I am
understanding things correctly.

I'm assuming the allocation being referred to is alloc_indirect_split(),
if so then it looks like it can fail and then we just fall back to using
the vring.desc directly which will work for my purposes as long as I limit
the capacity of the scatterlist to no more than the size of the vring.