linux-kernel - Re: [PATCH v6 1/2] Create debugfs file with virtio balloon usage information

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Date:   Mon, 1 Aug 2022 17:18:15 +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 v6 1/2] Create debugfs file with virtio balloon usage
 information

On 26.07.22 16:08, Alexander Atanasov wrote:
> Allow the guest to know how much it is ballooned by the host
> and how that memory is accounted.
> 
> It is useful when debugging out of memory conditions,
> as well for userspace processes that monitor the memory pressure
> and for nested virtualization.
> 
> Depending on the deflate on oom flag memory is accounted in two ways.
> If the flag is set the inflated pages are accounted as used memory.
> If the flag is not set the inflated pages are substracted from totalram.
> To make clear how are inflated pages accounted sign prefix the value.
> Where negative means substracted from totalram and positive means
> they are accounted as used.
> 
> Signed-off-by: Alexander Atanasov <alexander.atanasov@...tuozzo.com>
> ---
>  drivers/virtio/virtio_balloon.c | 59 +++++++++++++++++++++++++++++++++
>  1 file changed, 59 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
> V5:
>   - Made the calculatons work properly for both ways of memory accounting
>     with or without deflate_on_oom
>   - dropped comment
> V6:
>   - reduced the file to minimum
>   - unify accounting
> 
> I didn't understand if you agree to change the accounting to be the same
> so following part 2 is about it.
> 
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index f4c34a2a6b8e..97d3b29cb9f1 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 +732,59 @@ 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.
> + */
> +

Superfluous empty line. Personally, I'd just get rid of this
(comparatively obvious) documentation completely.

> +static int virtio_balloon_debug_show(struct seq_file *f, void *offset)
> +{
> +	struct virtio_balloon *vb = f->private;
> +	s64 num_pages = vb->num_pages << (VIRTIO_BALLOON_PFN_SHIFT - 10);

Rather call this "inflated_kb" then, it's no longer "pages".

> +
> +	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> +		num_pages = -num_pages;

With VIRTIO_BALLOON_F_DEFLATE_ON_OOM this will now always report "0".

which would be the same as "num_pages = 0;" and would deserve a comment
explaining why we don't indicate that as "inflated: ".

Thanks.

-- 
Thanks,

David / dhildenb