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Date:   Wed, 24 Jul 2019 16:43:39 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>, nitesh@redhat.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,
        lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com,
        pbonzini@redhat.com, dan.j.williams@intel.com
Subject: Re: [PATCH v2 5/5] virtio-balloon: Add support for providing page
 hints to host
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On Wed, Jul 24, 2019 at 01:37:47PM -0700, Alexander Duyck wrote:
> On Wed, 2019-07-24 at 15:02 -0400, 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 ... 
> 
> As per our discussion in the other patch I agree that we should either
> ignore the hint/report if page poisoning is enabled, or page poisoning
> should result in us poisoning the page when it is faulted back in. I had
> assumed we were doing the latter, I didn't realize that is was just
> disabling the free page hinting.

In fact I see that the latest versions of qemu don't seem to do
the later either. Need to fix that ASAP...


> > > ---
> > >  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 ...
> > 
> > > +	virtqueue_add_inbuf(vq, ph_dev_info->sg, num_hints, vb, GFP_KERNEL);
> > > +	virtqueue_kick(vq);
> > 
> > ... this will block forever.
> > 
> > > +	/* When host has read buffer, this completes via balloon_ack */
> > > +	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> > 
> > However below I suggest limiting capacity which will solve
> > this problem for you.
> 
> I wasn't aware that virtqueue_add_inbuf actually performed an allocation.
> 
> > > +}
> > > +
> > >  static void set_page_pfns(struct virtio_balloon *vb,
> > >  			  __virtio32 pfns[], struct page *page)
> > >  {
> > > @@ -476,6 +499,7 @@ static int init_vqs(struct virtio_balloon *vb)
> > >  	names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
> > >  	names[VIRTIO_BALLOON_VQ_STATS] = NULL;
> > >  	names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> > > +	names[VIRTIO_BALLOON_VQ_HINTING] = NULL;
> > >  
> > >  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> > >  		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
> > > @@ -487,11 +511,19 @@ static int init_vqs(struct virtio_balloon *vb)
> > >  		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> > >  	}
> > >  
> > > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
> > > +		names[VIRTIO_BALLOON_VQ_HINTING] = "hinting_vq";
> > > +		callbacks[VIRTIO_BALLOON_VQ_HINTING] = balloon_ack;
> > > +	}
> > > +
> > >  	err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
> > >  					 vqs, callbacks, names, NULL, NULL);
> > >  	if (err)
> > >  		return err;
> > >  
> > > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING))
> > > +		vb->hinting_vq = vqs[VIRTIO_BALLOON_VQ_HINTING];
> > > +
> > >  	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> > >  	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> > >  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> > > @@ -924,12 +956,24 @@ static int virtballoon_probe(struct virtio_device *vdev)
> > >  		if (err)
> > >  			goto out_del_balloon_wq;
> > >  	}
> > > +
> > > +	vb->ph_dev_info.react = virtballoon_page_hinting_react;
> > > +	vb->ph_dev_info.capacity = VIRTIO_BALLOON_ARRAY_HINTS_MAX;
> > 
> > As explained above I think you should limit this by vq size.
> > Otherwise virtqueue add buf might fail.
> > In fact by struct spec reading you need to limit it
> > anyway otherwise it will fail unconditionally.
> > In practice on most hypervisors it will typically work ...
> 
> So I would just need to query that via the virtqueue_get_vring_size
> function correct? I could probably just set capacity to the minimum of the
> HINTS_MAX and that value right?
> 
>