linux-kernel - Re: [PATCH v4 1/1] Create debugfs file with virtio balloon usage information

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Message-ID: <7fd5e645-3892-6e0d-de80-08728e29b150@redhat.com>
Date:   Thu, 14 Jul 2022 13:35:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     kernel@...nvz.org, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/1] Create debugfs file with virtio balloon usage
 information

On 05.07.22 10:36, 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.
> 
> Signed-off-by: Alexander Atanasov <alexander.atanasov@...tuozzo.com>
> ---
>  drivers/virtio/virtio_balloon.c     | 77 +++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_balloon.h |  1 +
>  2 files changed, 78 insertions(+)
> 
> V2:
>  - fixed coding style
>  - removed pretty print
> V3:
>  - removed dublicate of features
>  - comment about balooned_pages more clear
>  - convert host pages to balloon pages
> V4:
>  - added a define for BALLOON_PAGE_SIZE to make things clear
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index b9737da6c4dd..dc4ad584b947 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 +732,77 @@ static void report_free_page_func(struct work_struct *work)
>  	}
>  }
>  
> +/*
> + * DEBUGFS Interface
> + */
> +#ifdef CONFIG_DEBUG_FS
> +
> +#define guest_to_balloon_pages(i) ((i)*VIRTIO_BALLOON_PAGES_PER_PAGE)
> +/**
> + * 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: %d\n", "page_size", VIRTIO_BALLOON_PAGE_SIZE);
> +
> +	virtio_cread_le(b->vdev, struct virtio_balloon_config, actual,
> +			&num_pages);
> +	/*
> +	 * Pages allocated by host from the guest memory.
> +	 * Host inflates the balloon to get more memory.
> +	 * Guest needs to deflate the balloon to get more memory.
> +	 */

Please drop that comment. This is basic virtio-balloon operation that
must not be explained at this point.

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

totalram is calculated from totalram_pages().

When we inflate/deflate, we adjust totalram as well via
adjust_managed_page_count().

Consequently, this doesn't calculate what you actually want?

Total memory would be totalram+inflated, current would be totalram.


But, TBH, only export num_pages. User space can just lookup the other
information (totalram) via /proc/meminfo.

-- 
Thanks,

David / dhildenb