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

From: Alexander Atanasov
Date: Thu Jul 14 2022 - 09:20:32 EST


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@xxxxxxxxxxxxx>
drivers/virtio/virtio_balloon.c | 77 +++++++++++++++++++++++++++++
include/uapi/linux/virtio_balloon.h | 1 +
2 files changed, 78 insertions(+)

- fixed coding style
- removed pretty print
- removed dublicate of features
- comment about balooned_pages more clear
- convert host pages to balloon pages
- 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
+ */
+#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

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.


Alexander Atanasov