[PATCH 4/6] virtio-balloon: add drop cache support

[Liang Li]

virtio-balloon can make use of the amount of free memory to determine
the amount of memory to be filled in the balloon, but the amount of
free memory will be effected by the page cache, which can be reclaimed.
Drop the cache before getting the amount of free memory will be very
helpful to relect the exact amount of memroy that can be reclaimed.

This patch add a new feature to balloon driver to support this
operation, hypervisor can request the VM to drop it's cache, so as to
reclaim more memory.

Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
Cc: Amit Shah <amit.shah@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
---
 drivers/virtio/virtio_balloon.c     | 75 ++++++++++++++++++++++++++++++++++---
 include/uapi/linux/virtio_balloon.h |  1 +
 2 files changed, 71 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 1fa601b..5a30ca0 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -46,6 +46,15 @@ static int oom_pages = OOM_VBALLOON_DEFAULT_PAGES;
 module_param(oom_pages, int, S_IRUSR | S_IWUSR);
 MODULE_PARM_DESC(oom_pages, "pages to free on OOM");
 
+enum balloon_req_id {
+	BALLOON_DROP_CACHE,
+};
+
+struct balloon_req_hdr {
+	__virtio32 id;
+	__virtio32 param;
+};
+
 struct balloon_bmap_hdr {
 	__virtio32 id;
 	__virtio32 page_shift;
@@ -55,7 +64,7 @@ struct balloon_bmap_hdr {
 
 struct virtio_balloon {
 	struct virtio_device *vdev;
-	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
+	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *misc_vq;
 
 	/* The balloon servicing is delegated to a freezable workqueue. */
 	struct work_struct update_balloon_stats_work;
@@ -75,6 +84,7 @@ struct virtio_balloon {
 	unsigned long bmap_len;
 	/* Used to record the processed pfn range */
 	unsigned long min_pfn, max_pfn, start_pfn, end_pfn;
+	struct balloon_req_hdr req_hdr;
 	/*
 	 * The pages we've told the Host we're not using are enqueued
 	 * at vb_dev_info->pages list.
@@ -498,18 +508,62 @@ static void update_balloon_size_func(struct work_struct *work)
 		queue_work(system_freezable_wq, work);
 }
 
+static void misc_handle_rq(struct virtio_balloon *vb)
+{
+	struct virtqueue *vq;
+	struct scatterlist sg_out;
+	unsigned int len;
+	struct balloon_req_hdr *ptr_hdr;
+	struct scatterlist sg_in;
+
+	vq = vb->misc_vq;
+	ptr_hdr = virtqueue_get_buf(vq, &len);
+
+	if (!ptr_hdr || len != sizeof(vb->req_hdr))
+		return;
+
+	switch (ptr_hdr->id) {
+	case BALLOON_DROP_CACHE:
+#ifdef CONFIG_SYSCTL
+		drop_caches(ptr_hdr->param);
+#endif
+		sg_init_one(&sg_out, &ptr_hdr->id, sizeof(ptr_hdr->id));
+		virtqueue_add_outbuf(vq, &sg_out, 1, vb, GFP_KERNEL);
+		sg_init_one(&sg_in, &vb->req_hdr, sizeof(vb->req_hdr));
+		virtqueue_add_inbuf(vq, &sg_in, 1, &vb->req_hdr, GFP_KERNEL);
+		break;
+	default:
+		break;
+	}
+
+	virtqueue_kick(vq);
+}
+
+static void misc_request(struct virtqueue *vq)
+{
+	struct virtio_balloon *vb = vq->vdev->priv;
+
+	misc_handle_rq(vb);
+}
+
 static int init_vqs(struct virtio_balloon *vb)
 {
-	struct virtqueue *vqs[3];
-	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
-	static const char * const names[] = { "inflate", "deflate", "stats" };
+	struct virtqueue *vqs[4];
+	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack,
+					 stats_request, misc_request };
+	const char *names[] = { "inflate", "deflate", "stats", "misc" };
 	int err, nvqs;
 
 	/*
 	 * We expect two virtqueues: inflate and deflate, and
 	 * optionally stat.
 	 */
-	nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MISC))
+		nvqs = 4;
+	else
+		nvqs = virtio_has_feature(vb->vdev,
+					  VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
+
 	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names);
 	if (err)
 		return err;
@@ -530,6 +584,16 @@ static int init_vqs(struct virtio_balloon *vb)
 			BUG();
 		virtqueue_kick(vb->stats_vq);
 	}
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MISC)) {
+		struct scatterlist sg_in;
+
+		vb->misc_vq = vqs[3];
+		sg_init_one(&sg_in, &vb->req_hdr, sizeof(vb->req_hdr));
+		if (virtqueue_add_inbuf(vb->misc_vq, &sg_in, 1,
+		    &vb->req_hdr, GFP_KERNEL) < 0)
+			BUG();
+		virtqueue_kick(vb->misc_vq);
+	}
 	return 0;
 }
 
@@ -725,6 +789,7 @@ static unsigned int features[] = {
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
 	VIRTIO_BALLOON_F_PAGE_BITMAP,
+	VIRTIO_BALLOON_F_MISC,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index f78fa47..5a7309d 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -35,6 +35,7 @@
 #define VIRTIO_BALLOON_F_STATS_VQ	1 /* Memory Stats virtqueue */
 #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
 #define VIRTIO_BALLOON_F_PAGE_BITMAP	3 /* Send page info with bitmap */
+#define VIRTIO_BALLOON_F_MISC		4 /* Send request and get misc info */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
-- 
1.9.1