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 <[email protected]> > > > > > > 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 <[email protected]> > > 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.
