Linux Virtualization - Aug 2015 - [PATCH 0/2] virtio_balloon: do not change memory amount visible via /proc/meminfo

Though there is a problem in this setup. The end-user and hosting provider
have signed SLA agreement in which some amount of memory is guaranted for
the guest. The good thing is that this memory will be given to the guest
when the guest will really need it (f.e. with OOM in guest and with
VIRTIO_BALLOON_F_DEFLATE_ON_OOM configuration flag set). The bad thing
is that end-user does not know this.

Balloon by default reduce the amount of memory exposed to the end-user
each time when the page is stolen from guest or returned back by using
adjust_managed_page_count and thus /proc/meminfo shows reduced amount
of memory.

Fortunately the solution is simple, we should just avoid to call
adjust_managed_page_count with VIRTIO_BALLOON_F_DEFLATE_ON_OOM set.

Please note that neither VMWare ballon nor HyperV balloon do not care
about proper handling of adjust_managed_page_count at all.

Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Michael S. Tsirkin <mst at redhat.com>

and rename it to release_pages_balloon. The function originally takes
arrays of pfns and now it takes pointer to struct virtio_ballon.
This change is necessary to conditionally call adjust_managed_page_count
in the next patch.

Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/virtio/virtio_balloon.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 82e80e0..8543c9a 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -166,13 +166,13 @@ static void fill_balloon(struct virtio_balloon *vb, size_t
num)
 	mutex_unlock(&vb->balloon_lock);
 }
 
-static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
+static void release_pages_balloon(struct virtio_balloon *vb)
 {
 	unsigned int i;
 
 	/* Find pfns pointing at start of each page, get pages and free them. */
-	for (i = 0; i < num; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
-		struct page *page = balloon_pfn_to_page(pfns[i]);
+	for (i = 0; i < vb->num_pfns; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+		struct page *page = balloon_pfn_to_page(vb->pfns[i]);
 		adjust_managed_page_count(page, 1);
 		put_page(page); /* balloon reference */
 	}
@@ -206,7 +206,7 @@ static unsigned leak_balloon(struct virtio_balloon *vb,
size_t num)
 	if (vb->num_pfns != 0)
 		tell_host(vb, vb->deflate_vq);
 	mutex_unlock(&vb->balloon_lock);
-	release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	release_pages_balloon(vb);
 	return num_freed_pages;
 }
 
-- 
2.1.4

Balloon device is frequently used as a mean of cooperative memory control
in between guest and host to manage memory overcommitment. This is the
typical case for any hosting workload when KVM guest is provided for
end-user.

Though there is a problem in this setup. The end-user and hosting provider
have signed SLA agreement in which some amount of memory is guaranted for
the guest. The good thing is that this memory will be given to the guest
when the guest will really need it (f.e. with OOM in guest and with
VIRTIO_BALLOON_F_DEFLATE_ON_OOM configuration flag set). The bad thing
is that end-user does not know this.

Balloon by default reduce the amount of memory exposed to the end-user
each time when the page is stolen from guest or returned back by using
adjust_managed_page_count and thus /proc/meminfo shows reduced amount
of memory.

Fortunately the solution is simple, we should just avoid to call
adjust_managed_page_count with VIRTIO_BALLOON_F_DEFLATE_ON_OOM set.

Signed-off-by: Denis V. Lunev <den at openvz.org>
CC: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/virtio/virtio_balloon.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 8543c9a..7efc329 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -157,7 +157,9 @@ static void fill_balloon(struct virtio_balloon *vb, size_t
num)
 		}
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
-		adjust_managed_page_count(page, -1);
+		if (!virtio_has_feature(vb->vdev,
+					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+			adjust_managed_page_count(page, -1);
 	}
 
 	/* Did we get any? */
@@ -173,7 +175,9 @@ static void release_pages_balloon(struct virtio_balloon *vb)
 	/* Find pfns pointing at start of each page, get pages and free them. */
 	for (i = 0; i < vb->num_pfns; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		struct page *page = balloon_pfn_to_page(vb->pfns[i]);
-		adjust_managed_page_count(page, 1);
+		if (!virtio_has_feature(vb->vdev,
+					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+			adjust_managed_page_count(page, 1);
 		put_page(page); /* balloon reference */
 	}
 }
-- 
2.1.4