[PATCH RFC] virtio_balloon: conservative balloon page shrinking

[Wei Wang <wei.w.wang@intel.com>]

There are cases that users want to shrink balloon pages after the
pagecache depleted. The conservative_shrinker lets the shrinker
shrink balloon pages when all the pagecache has been reclaimed.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
---
 drivers/virtio/virtio_balloon.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 93f995f6cf36..b4c5bb13a867 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -42,6 +42,10 @@
 static struct vfsmount *balloon_mnt;
 #endif
 
+static bool conservative_shrinker = true;
+module_param(conservative_shrinker, bool, 0644);
+MODULE_PARM_DESC(conservative_shrinker, "conservatively shrink balloon pages");
+
 enum virtio_balloon_vq {
 	VIRTIO_BALLOON_VQ_INFLATE,
 	VIRTIO_BALLOON_VQ_DEFLATE,
@@ -796,6 +800,10 @@ static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
 {
 	unsigned long pages_freed = 0;
 
+	/* Balloon pages only gets shrunk when the pagecache depleted */
+	if (conservative_shrinker && global_node_page_state(NR_FILE_PAGES))
+		return 0;
+
 	/*
 	 * One invocation of leak_balloon can deflate at most
 	 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
@@ -837,7 +845,11 @@ static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
 					struct virtio_balloon, shrinker);
 	unsigned long count;
 
-	count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
+	if (conservative_shrinker && global_node_page_state(NR_FILE_PAGES))
+		count = 0;
+	else
+		count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
+
 	count += vb->num_free_page_blocks * VIRTIO_BALLOON_HINT_BLOCK_PAGES;
 
 	return count;
-- 
2.17.1