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Date:   Mon, 27 Jun 2022 16:42:56 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Atanasov <alexander.atanasov@virtuozzo.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Jason Wang <jasowang@redhat.com>, kernel@openvz.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] Create debugfs file with virtio balloon usage
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References: <20220627085838-mutt-send-email-mst@kernel.org>
 <20220627191910.22734-1-alexander.atanasov@virtuozzo.com>
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On Mon, Jun 27, 2022 at 07:19:09PM +0000, Alexander Atanasov wrote:
> Allow the guest to know how much it is ballooned by the host.
> It is useful when debugging out of memory conditions.
> 
> When host gets back memory from the guest it is accounted
> as used memory in the guest but the guest have no way to know
> how much it is actually ballooned.
> 
> No pretty printing and fixed as per coding style.
> 
> Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
> ---
>  drivers/virtio/virtio_balloon.c | 71 +++++++++++++++++++++++++++++++++
>  1 file changed, 71 insertions(+)
> 
> - Leave pretty print to userspace
> - Fixed coding style
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index b9737da6c4dd..1bb6a6d0e37b 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -10,6 +10,7 @@
>  #include <linux/virtio_balloon.h>
>  #include <linux/swap.h>
>  #include <linux/workqueue.h>
> +#include <linux/debugfs.h>
>  #include <linux/delay.h>
>  #include <linux/slab.h>
>  #include <linux/module.h>
> @@ -731,6 +732,71 @@ static void report_free_page_func(struct work_struct *work)
>  	}
>  }
>  
> +/*
> + * DEBUGFS Interface
> + */
> +#ifdef CONFIG_DEBUG_FS
> +
> +/**
> + * virtio_balloon_debug_show - shows statistics of balloon operations.
> + * @f: pointer to the &struct seq_file.
> + * @offset: ignored.
> + *
> + * Provides the statistics that can be accessed in virtio-balloon in the debugfs.
> + *
> + * Return: zero on success or an error code.
> + */
> +static int virtio_balloon_debug_show(struct seq_file *f, void *offset)
> +{
> +	struct virtio_balloon *b = f->private;
> +	u32 num_pages;
> +	struct sysinfo i;
> +
> +	si_meminfo(&i);
> +
> +	seq_printf(f, "%-22s: %llx\n", "capabilities", b->vdev->features);

why do we need this in debugfs? Isn't this available in sysfs already?

> +	seq_printf(f, "%-22s: %d\n", "page_size", 4096);

I suspect this function doesn't work properly when page size is not 4K.

> +
> +	virtio_cread_le(b->vdev, struct virtio_balloon_config, actual,
> +			&num_pages);
> +	/* Memory returned to host or amount we can inflate if possible */
> +	seq_printf(f, "%-22s: %u\n", "ballooned_pages", num_pages);

I don't really get the comment here.

> +
> +	/* Total Memory for the guest from host */
> +	seq_printf(f, "%-22s: %lu\n", "total_pages", i.totalram);
> +
> +	/* Current memory for the guest */
> +	seq_printf(f, "%-22s: %lu\n", "current_pages", i.totalram - num_pages);

Are you sure these are in units of 4Kbyte pages?

> +	return 0;
> +}
> +
> +DEFINE_SHOW_ATTRIBUTE(virtio_balloon_debug);
> +
> +static void  virtio_balloon_debugfs_init(struct virtio_balloon *b)
> +{
> +	debugfs_create_file("virtio-balloon", 0444, NULL, b,
> +			    &virtio_balloon_debug_fops);
> +}
> +
> +static void  virtio_balloon_debugfs_exit(struct virtio_balloon *b)
> +{
> +	debugfs_remove(debugfs_lookup("virtio-balloon", NULL));
> +}
> +
> +#else
> +
> +static inline void virtio_balloon_debugfs_init(struct virtio_balloon *b)
> +{
> +}
> +
> +static inline void virtio_balloon_debugfs_exit(struct virtio_balloon *b)
> +{
> +}
> +
> +#endif	/* CONFIG_DEBUG_FS */
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf of
> @@ -1019,6 +1085,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  
>  	if (towards_target(vb))
>  		virtballoon_changed(vdev);
> +
> +	virtio_balloon_debugfs_init(vb);
> +
>  	return 0;
>  
>  out_unregister_oom:
> @@ -1065,6 +1134,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> +	virtio_balloon_debugfs_exit(vb);
> +
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
>  		page_reporting_unregister(&vb->pr_dev_info);
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> -- 
> 2.25.1