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Date: Thu, 25 Jul 2019 10:59:40 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc: Nitesh Narayan Lal <nitesh@...hat.com>,
Alexander Duyck <alexander.duyck@...il.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 Thu, Jul 25, 2019 at 07:56:15AM -0700, Alexander Duyck wrote:
> 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@...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 ...
> > >
> > >
> > > > ---
> > > > 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.
>
Right. And maybe tweak the GFP mask - no reason to try to
allocate memory aggressively with just 1 element in flight.
>
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