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

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Date:   Thu, 14 Jul 2022 16:20:17 +0300
From:   Alexander Atanasov <alexander.atanasov@...tuozzo.com>
To:     David Hildenbrand <david@...hat.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

Hello,

On 14/07/2022 14:35, David Hildenbrand wrote:
> 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.

Ok

>> +	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().

That is true only when not using DEFLATE_ON_OOM.

Otherwise inflated memory is accounted as used and total ram stays the same.
> Consequently, this doesn't calculate what you actually want?
> Total memory would be totalram+inflated, current would be totalram.

My calculations are correct for the case deflate_on_oom is enabled.

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

I have missed that the memory accounting is made differently depending 
on a flag.

Since the calculations are different i'd prefer to have the values 
calculate and printed there.

Where no matter what the flags is:

  - ballooned_pages     -> how much is balloon inflated
  - total_pages             -> total ram the guest can get
  - current_pages         -> current ram the guest have

Patch to unify both cases with or without deflate on oom coming next.

-- 

Regards,
Alexander Atanasov