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Date: Wed, 24 Jul 2019 15:07:42 -0400
From: Nitesh Narayan Lal <nitesh@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>,
Alexander Duyck <alexander.duyck@...il.com>
Cc: 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,
alexander.h.duyck@...ux.intel.com
Subject: Re: [PATCH v2 5/5] virtio-balloon: Add support for providing page
hints to host
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
Once VIRTIO_BALLOON_F_PAGE_POISON user side support is available.
Can't we just use that at the time of initialization?
> 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 ...
>
>> + 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.
>
>
>
>> +}
>> +
>> 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 ...
>
>> + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
>> + err = page_hinting_startup(&vb->ph_dev_info);
>> + if (err)
>> + goto out_unregister_shrinker;
>> + }
>> +
>> virtio_device_ready(vdev);
>>
>> if (towards_target(vb))
>> virtballoon_changed(vdev);
>> return 0;
>>
>> +out_unregister_shrinker:
>> + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
>> + virtio_balloon_unregister_shrinker(vb);
>> out_del_balloon_wq:
>> if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>> destroy_workqueue(vb->balloon_wq);
>> @@ -958,6 +1002,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
>> {
>> struct virtio_balloon *vb = vdev->priv;
>>
>> + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING))
>> + page_hinting_shutdown(&vb->ph_dev_info);
>> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
>> virtio_balloon_unregister_shrinker(vb);
>> spin_lock_irq(&vb->stop_update_lock);
>> @@ -1027,6 +1073,7 @@ static int virtballoon_validate(struct virtio_device *vdev)
>> VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
>> VIRTIO_BALLOON_F_FREE_PAGE_HINT,
>> VIRTIO_BALLOON_F_PAGE_POISON,
>> + VIRTIO_BALLOON_F_HINTING,
>> };
>>
>> static struct virtio_driver virtio_balloon_driver = {
>> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
>> index a1966cd7b677..2b0f62814e22 100644
>> --- a/include/uapi/linux/virtio_balloon.h
>> +++ b/include/uapi/linux/virtio_balloon.h
>> @@ -36,6 +36,7 @@
>> #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM 2 /* Deflate balloon on OOM */
>> #define VIRTIO_BALLOON_F_FREE_PAGE_HINT 3 /* VQ to report free pages */
>> #define VIRTIO_BALLOON_F_PAGE_POISON 4 /* Guest is using page poisoning */
>> +#define VIRTIO_BALLOON_F_HINTING 5 /* Page hinting virtqueue */
>>
>> /* Size of a PFN in the balloon interface. */
>> #define VIRTIO_BALLOON_PFN_SHIFT 12
--
Thanks
Nitesh
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