Re: [Qemu-devel] [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

[Michael S. Tsirkin]

Looks good to me. minor comments below.

On Sat, Sep 30, 2017 at 12:05:52PM +0800, Wei Wang wrote:
> @@ -141,13 +146,128 @@ static void set_page_pfns(struct virtio_balloon *vb,
>                                         page_to_balloon_pfn(page) + i);
>  }
>  
> +
> +static void kick_and_wait(struct virtqueue *vq, wait_queue_head_t wq_head)
> +{
> +     unsigned int len;
> +
> +     virtqueue_kick(vq);
> +     wait_event(wq_head, virtqueue_get_buf(vq, &len));
> +}
> +
> +static int add_one_sg(struct virtqueue *vq, void *addr, uint32_t size)
> +{
> +     struct scatterlist sg;
> +     unsigned int len;
> +
> +     sg_init_one(&sg, addr, size);
> +
> +     /* Detach all the used buffers from the vq */
> +     while (virtqueue_get_buf(vq, &len))
> +             ;
> +
> +     return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> +}
> +
> +static int send_balloon_page_sg(struct virtio_balloon *vb,
> +                              struct virtqueue *vq,
> +                              void *addr,
> +                              uint32_t size,
> +                              bool batch)
> +{
> +     int err;
> +
> +     err = add_one_sg(vq, addr, size);
> +
> +     /* If batchng is requested, we batch till the vq is full */

typo

> +     if (!batch || !vq->num_free)
> +             kick_and_wait(vq, vb->acked);
> +
> +     return err;
> +}

If add_one_sg fails, kick_and_wait will hang forever.

The reason this might work in because
1. with 1 sg there are no memory allocations
2. if adding fails on vq full, then something
   is in queue and will wake up kick_and_wait.

So in short this is expected to never fail.
How about a BUG_ON here then?
And make it void, and add a comment with above explanation.

> +
> +/*
> + * Send balloon pages in sgs to host. The balloon pages are recorded in the
> + * page xbitmap. Each bit in the bitmap corresponds to a page of PAGE_SIZE.
> + * The page xbitmap is searched for continuous "1" bits, which correspond
> + * to continuous pages, to chunk into sgs.
> + *
> + * @page_xb_start and @page_xb_end form the range of bits in the xbitmap that
> + * need to be searched.
> + */
> +static void tell_host_sgs(struct virtio_balloon *vb,
> +                       struct virtqueue *vq,
> +                       unsigned long page_xb_start,
> +                       unsigned long page_xb_end)
> +{
> +     unsigned long sg_pfn_start, sg_pfn_end;
> +     void *sg_addr;
> +     uint32_t sg_len, sg_max_len = round_down(UINT_MAX, PAGE_SIZE);
> +     int err = 0;
> +
> +     sg_pfn_start = page_xb_start;
> +     while (sg_pfn_start < page_xb_end) {
> +             sg_pfn_start = xb_find_next_set_bit(&vb->page_xb, sg_pfn_start,
> +                                                 page_xb_end);
> +             if (sg_pfn_start == page_xb_end + 1)
> +                     break;
> +             sg_pfn_end = xb_find_next_zero_bit(&vb->page_xb,
> +                                                sg_pfn_start + 1,
> +                                                page_xb_end);
> +             sg_addr = (void *)pfn_to_kaddr(sg_pfn_start);
> +             sg_len = (sg_pfn_end - sg_pfn_start) << PAGE_SHIFT;
> +             while (sg_len > sg_max_len) {
> +                     err = send_balloon_page_sg(vb, vq, sg_addr, sg_max_len,
> +                                                true);
> +                     if (unlikely(err < 0))
> +                             goto err_out;
> +                     sg_addr += sg_max_len;
> +                     sg_len -= sg_max_len;
> +             }
> +             err = send_balloon_page_sg(vb, vq, sg_addr, sg_len, true);
> +             if (unlikely(err < 0))
> +                     goto err_out;
> +             sg_pfn_start = sg_pfn_end + 1;
> +     }
> +
> +     /*
> +      * The last few sgs may not reach the batch size, but need a kick to
> +      * notify the device to handle them.
> +      */
> +     if (vq->num_free != virtqueue_get_vring_size(vq))
> +             kick_and_wait(vq, vb->acked);
> +
> +     xb_clear_bit_range(&vb->page_xb, page_xb_start, page_xb_end);
> +     return;
> +
> +err_out:
> +     dev_warn(&vb->vdev->dev, "%s failure: %d\n", __func__, err);

so fundamentally just make send_balloon_page_sg void then.

> +}
> +
> +static inline void xb_set_page(struct virtio_balloon *vb,
> +                            struct page *page,
> +                            unsigned long *pfn_min,
> +                            unsigned long *pfn_max)
> +{
> +     unsigned long pfn = page_to_pfn(page);
> +
> +     *pfn_min = min(pfn, *pfn_min);
> +     *pfn_max = max(pfn, *pfn_max);
> +     xb_preload(GFP_KERNEL);
> +     xb_set_bit(&vb->page_xb, pfn);
> +     xb_preload_end();
> +}
> +
>  static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
>  {
>       struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
>       unsigned num_allocated_pages;
> +     bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
> +     unsigned long pfn_max = 0, pfn_min = ULONG_MAX;
>  
>       /* We can only do one array worth at a time. */
> -     num = min(num, ARRAY_SIZE(vb->pfns));
> +     if (!use_sg)
> +             num = min(num, ARRAY_SIZE(vb->pfns));
>  
>       mutex_lock(&vb->balloon_lock);
>       for (vb->num_pfns = 0; vb->num_pfns < num;
> @@ -162,7 +282,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
> size_t num)
>                       msleep(200);
>                       break;
>               }
> -             set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
> +             if (use_sg)
> +                     xb_set_page(vb, page, &pfn_min, &pfn_max);
> +             else
> +                     set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
>               vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
>               if (!virtio_has_feature(vb->vdev,
>                                       VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> @@ -171,8 +296,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
> size_t num)
>  
>       num_allocated_pages = vb->num_pfns;
>       /* Did we get any? */
> -     if (vb->num_pfns != 0)
> -             tell_host(vb, vb->inflate_vq);
> +     if (vb->num_pfns) {
> +             if (use_sg)
> +                     tell_host_sgs(vb, vb->inflate_vq, pfn_min, pfn_max);
> +             else
> +                     tell_host(vb, vb->inflate_vq);
> +     }
>       mutex_unlock(&vb->balloon_lock);
>  
>       return num_allocated_pages;
> @@ -198,9 +327,12 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
> size_t num)
>       struct page *page;
>       struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
>       LIST_HEAD(pages);
> +     bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
> +     unsigned long pfn_max = 0, pfn_min = ULONG_MAX;
>  
> -     /* We can only do one array worth at a time. */
> -     num = min(num, ARRAY_SIZE(vb->pfns));
> +     /* Traditionally, we can only do one array worth at a time. */
> +     if (!use_sg)
> +             num = min(num, ARRAY_SIZE(vb->pfns));
>  
>       mutex_lock(&vb->balloon_lock);
>       /* We can't release more pages than taken */
> @@ -210,7 +342,11 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
> size_t num)
>               page = balloon_page_dequeue(vb_dev_info);
>               if (!page)
>                       break;
> -             set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +             if (use_sg)
> +                     xb_set_page(vb, page, &pfn_min, &pfn_max);
> +             else
> +                     set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> +
>               list_add(&page->lru, &pages);
>               vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
>       }
> @@ -221,8 +357,12 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
> size_t num)
>        * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
>        * is true, we *have* to do it in this order
>        */
> -     if (vb->num_pfns != 0)
> -             tell_host(vb, vb->deflate_vq);
> +     if (vb->num_pfns) {
> +             if (use_sg)
> +                     tell_host_sgs(vb, vb->deflate_vq, pfn_min, pfn_max);
> +             else
> +                     tell_host(vb, vb->deflate_vq);
> +     }
>       release_pages_balloon(vb, &pages);
>       mutex_unlock(&vb->balloon_lock);
>       return num_freed_pages;
> @@ -441,6 +581,7 @@ static int init_vqs(struct virtio_balloon *vb)
>  }
>  
>  #ifdef CONFIG_BALLOON_COMPACTION
> +
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf of
>   *                        a compation thread.     (called under page lock)
> @@ -464,6 +605,7 @@ static int virtballoon_migratepage(struct 
> balloon_dev_info *vb_dev_info,
>  {
>       struct virtio_balloon *vb = container_of(vb_dev_info,
>                       struct virtio_balloon, vb_dev_info);
> +     bool use_sg = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_SG);
>       unsigned long flags;
>  
>       /*
> @@ -485,16 +627,24 @@ static int virtballoon_migratepage(struct 
> balloon_dev_info *vb_dev_info,
>       vb_dev_info->isolated_pages--;
>       __count_vm_event(BALLOON_MIGRATE);
>       spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> -     vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -     set_page_pfns(vb, vb->pfns, newpage);
> -     tell_host(vb, vb->inflate_vq);
> -
> +     if (use_sg) {
> +             send_balloon_page_sg(vb, vb->inflate_vq, page_address(newpage),
> +                                  PAGE_SIZE, false);
> +     } else {
> +             vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +             set_page_pfns(vb, vb->pfns, newpage);
> +             tell_host(vb, vb->inflate_vq);
> +     }
>       /* balloon's page migration 2nd step -- deflate "page" */
>       balloon_page_delete(page);
> -     vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> -     set_page_pfns(vb, vb->pfns, page);
> -     tell_host(vb, vb->deflate_vq);
> -
> +     if (use_sg) {
> +             send_balloon_page_sg(vb, vb->deflate_vq, page_address(page),
> +                                  PAGE_SIZE, false);
> +     } else {
> +             vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +             set_page_pfns(vb, vb->pfns, page);
> +             tell_host(vb, vb->deflate_vq);
> +     }
>       mutex_unlock(&vb->balloon_lock);
>  
>       put_page(page); /* balloon reference */
> @@ -553,6 +703,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
>       if (err)
>               goto out_free_vb;
>  
> +     if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_SG))
> +             xb_init(&vb->page_xb);
> +
>       vb->nb.notifier_call = virtballoon_oom_notify;
>       vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
>       err = register_oom_notifier(&vb->nb);
> @@ -669,6 +822,7 @@ static unsigned int features[] = {
>       VIRTIO_BALLOON_F_MUST_TELL_HOST,
>       VIRTIO_BALLOON_F_STATS_VQ,
>       VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> +     VIRTIO_BALLOON_F_SG,
>  };
>  
>  static struct virtio_driver virtio_balloon_driver = {
> diff --git a/include/uapi/linux/virtio_balloon.h 
> b/include/uapi/linux/virtio_balloon.h
> index 343d7dd..37780a7 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -34,6 +34,7 @@
>  #define VIRTIO_BALLOON_F_MUST_TELL_HOST      0 /* Tell before reclaiming 
> pages */
>  #define VIRTIO_BALLOON_F_STATS_VQ    1 /* Memory Stats virtqueue */
>  #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM      2 /* Deflate balloon on OOM */
> +#define VIRTIO_BALLOON_F_SG          3 /* Use sg instead of PFN lists */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
> -- 
> 2.7.4