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Date: Wed, 24 Jul 2019 13:37:47 -0700
From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
Alexander Duyck <alexander.duyck@...il.com>
Cc: nitesh@...hat.com, kvm@...r.kernel.org, david@...hat.com,
dave.hansen@...el.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, akpm@...ux-foundation.org,
yang.zhang.wz@...il.com, pagupta@...hat.com, riel@...riel.com,
konrad.wilk@...cle.com, lcapitulino@...hat.com,
wei.w.wang@...el.com, aarcange@...hat.com, pbonzini@...hat.com,
dan.j.williams@...el.com
Subject: Re: [PATCH v2 5/5] virtio-balloon: Add support for providing page
hints to host
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@...ux.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@...ux.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.
> > ---
> > 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?
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