Date:   Tue, 4 Jun 2019 07:50:49 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][Patch v10 2/2] virtio-balloon: page_hinting: reporting to
 the host


On 6/4/19 3:12 AM, David Hildenbrand wrote:
> On 04.06.19 00:38, Alexander Duyck wrote:
>> On Mon, Jun 3, 2019 at 10:04 AM Nitesh Narayan Lal <nitesh@...hat.com> wrote:
>>> Enables the kernel to negotiate VIRTIO_BALLOON_F_HINTING feature with the
>>> host. If it is available and page_hinting_flag is set to true, page_hinting
>>> is enabled and its callbacks are configured along with the max_pages count
>>> which indicates the maximum number of pages that can be isolated and hinted
>>> at a time. Currently, only free pages of order >= (MAX_ORDER - 2) are
>>> reported. To prevent any false OOM max_pages count is set to 16.
>>>
>>> By default page_hinting feature is enabled and gets loaded as soon
>>> as the virtio-balloon driver is loaded. However, it could be disabled
>>> by writing the page_hinting_flag which is a virtio-balloon parameter.
>>>
>>> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
>>> ---
>>>  drivers/virtio/virtio_balloon.c     | 112 +++++++++++++++++++++++++++-
>>>  include/uapi/linux/virtio_balloon.h |  14 ++++
>>>  2 files changed, 125 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>>> index f19061b585a4..40f09ea31643 100644
>>> --- a/drivers/virtio/virtio_balloon.c
>>> +++ b/drivers/virtio/virtio_balloon.c
>>> @@ -31,6 +31,7 @@
>>>  #include <linux/mm.h>
>>>  #include <linux/mount.h>
>>>  #include <linux/magic.h>
>>> +#include <linux/page_hinting.h>
>>>
>>>  /*
>>>   * Balloon device works in 4K page units.  So each page is pointed to by
>>> @@ -48,6 +49,7 @@
>>>  /* The size of a free page block in bytes */
>>>  #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
>>>         (1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
>>> +#define VIRTIO_BALLOON_PAGE_HINTING_MAX_PAGES  16
>>>
>>>  #ifdef CONFIG_BALLOON_COMPACTION
>>>  static struct vfsmount *balloon_mnt;
>>> @@ -58,6 +60,7 @@ 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
>>>  };
>>>
>>> @@ -67,7 +70,8 @@ enum virtio_balloon_config_read {
>>>
>>>  struct virtio_balloon {
>>>         struct virtio_device *vdev;
>>> -       struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
>>> +       struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq,
>>> +                        *hinting_vq;
>>>
>>>         /* Balloon's own wq for cpu-intensive work items */
>>>         struct workqueue_struct *balloon_wq;
>>> @@ -125,6 +129,9 @@ struct virtio_balloon {
>>>
>>>         /* To register a shrinker to shrink memory upon memory pressure */
>>>         struct shrinker shrinker;
>>> +
>>> +       /* object pointing at the array of isolated pages ready for hinting */
>>> +       struct hinting_data *hinting_arr;
>> Just make this an array of size VIRTIO_BALLOON_PAGE_HINTING_MAX_PAGES.
>> It will save a bunch of complexity later.
> +1
>
> [...]
>>> +struct virtio_balloon *hvb;
>>> +bool page_hinting_flag = true;
>>> +module_param(page_hinting_flag, bool, 0444);
>>> +MODULE_PARM_DESC(page_hinting_flag, "Enable page hinting");
>>> +
>>> +static bool virtqueue_kick_sync(struct virtqueue *vq)
>>> +{
>>> +       u32 len;
>>> +
>>> +       if (likely(virtqueue_kick(vq))) {
>>> +               while (!virtqueue_get_buf(vq, &len) &&
>>> +                      !virtqueue_is_broken(vq))
>>> +                       cpu_relax();
>>> +               return true;
>> Is this a synchronous setup? It seems kind of wasteful to have a
>> thread busy waiting here like this. It might make more sense to just
>> make this work like the other balloon queues and have a wait event
>> with a wake up in the interrupt handler for the queue.
> +1
>
> [...]
>
>>> +       gpaddr = virt_to_phys(hvb->hinting_arr);
>>> +       hint_req->phys_addr = cpu_to_virtio64(hvb->vdev, gpaddr);
>>> +       hint_req->size = cpu_to_virtio32(hvb->vdev, entries);
>>> +       sg_init_one(&sg, hint_req, sizeof(*hint_req));
>>> +       err = virtqueue_add_outbuf(vq, &sg, 1, hint_req, GFP_KERNEL);
>>> +       if (!err)
>>> +               virtqueue_kick_sync(hvb->hinting_vq);
>>> +
>>> +       kfree(hint_req);
>>> +}
>>> +
>>> +int page_hinting_prepare(void)
>>> +{
>>> +       hvb->hinting_arr = kmalloc_array(VIRTIO_BALLOON_PAGE_HINTING_MAX_PAGES,
>>> +                                        sizeof(*hvb->hinting_arr), GFP_KERNEL);
>>> +       if (!hvb->hinting_arr)
>>> +               return -ENOMEM;
>>> +       return 0;
>>> +}
>>> +
>> Why make the hinting_arr a dynamic allocation? You should probably
>> just make it a static array within the virtio_balloon structure. Then
>> you don't have the risk of an allocation failing and messing up the
>> hints.
> +1
>
>>> +void hint_pages(struct list_head *pages)
>>> +{
>>> +       struct page *page, *next;
>>> +       unsigned long pfn;
>>> +       int idx = 0, order;
>>> +
>>> +       list_for_each_entry_safe(page, next, pages, lru) {
>>> +               pfn = page_to_pfn(page);
>>> +               order = page_private(page);
>>> +               hvb->hinting_arr[idx].phys_addr = pfn << PAGE_SHIFT;
>>> +               hvb->hinting_arr[idx].size = (1 << order) * PAGE_SIZE;
>>> +               idx++;
>>> +       }
>>> +       page_hinting_report(idx);
>>> +}
>>> +
>> Getting back to my suggestion from earlier today. It might make sense
>> to not bother with the PAGE_SHIFT or PAGE_SIZE multiplication if you
>> just record everything in VIRTIO_BALLOON_PAGES intead of using the
>> actual address and size.
> I think I prefer "addr + size".
>
>> Same comment here. Make this array a part of virtio_balloon and you
>> don't have to free it.
>>
>>> +static const struct page_hinting_cb hcb = {
>>> +       .prepare = page_hinting_prepare,
>>> +       .hint_pages = hint_pages,
>>> +       .cleanup = page_hinting_cleanup,
>>> +       .max_pages = VIRTIO_BALLOON_PAGE_HINTING_MAX_PAGES,
>>> +};
>> With the above changes prepare and cleanup can be dropped.
> +1
>
>>> +#endif
>>> +
>>>  static u32 page_to_balloon_pfn(struct page *page)
>>>  {
>>>         unsigned long pfn = page_to_pfn(page);
>>> @@ -488,6 +574,7 @@ 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";
>>> @@ -499,11 +586,18 @@ 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] = NULL;
>>> +       }
>>>         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)) {
>>> @@ -942,6 +1036,14 @@ static int virtballoon_probe(struct virtio_device *vdev)
>>>                 if (err)
>>>                         goto out_del_balloon_wq;
>>>         }
>>> +
>>> +#ifdef CONFIG_PAGE_HINTING
>>> +       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING) &&
>>> +           page_hinting_flag) {
>>> +               hvb = vb;
>>> +               page_hinting_enable(&hcb);
>>> +       }
>>> +#endif
>>>         virtio_device_ready(vdev);
>>>
>>>         if (towards_target(vb))
>>> @@ -989,6 +1091,12 @@ static void virtballoon_remove(struct virtio_device *vdev)
>>>                 destroy_workqueue(vb->balloon_wq);
>>>         }
>>>
>>> +#ifdef CONFIG_PAGE_HINTING
>>> +       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
> Nitesh, you should only disable if you actually enabled it
> (page_hinting_flag).
+1, thanks.
>
>
-- 
Regards
Nitesh



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