[PATCH] KVM simplified virtio balloon driver

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

After discussions with Anthony Liguori, it seems that the virtio
balloon can be made even simpler.  Here's my attempt.

Since the balloon requires Guest cooperation anyway, there seems
little reason to force it to tell the Host when it wants to reuse a
page.  It can simply fault it in.

Moreover, the target is best expressed in balloon size, since there is
no portable way of getting the total RAM in the system.  The host can
do the math.

Tested with a (fairly hacky) lguest patch.

Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
---
 drivers/virtio/Kconfig          |   10 +
 drivers/virtio/Makefile         |    1
 drivers/virtio/virtio_balloon.c |  230 ++++++++++++++++++++++++++++++++++++++++
 include/linux/virtio_balloon.h  |   13 ++
 4 files changed, 254 insertions(+)

diff -r c4762959de25 drivers/virtio/Kconfig
--- a/drivers/virtio/Kconfig	Thu Jan 17 10:31:37 2008 +1100
+++ b/drivers/virtio/Kconfig	Thu Jan 17 12:28:23 2008 +1100
@@ -23,3 +23,13 @@ config VIRTIO_PCI
 
 	  If unsure, say M.
 
+config VIRTIO_BALLOON
+	tristate "Virtio balloon driver (EXPERIMENTAL)"
+	select VIRTIO
+	select VIRTIO_RING
+	---help---
+	 This driver supports increasing and decreasing the amount 
+	 of memory within a KVM guest.
+
+	 If unsure, say M.
+
diff -r c4762959de25 drivers/virtio/Makefile
--- a/drivers/virtio/Makefile	Thu Jan 17 10:31:37 2008 +1100
+++ b/drivers/virtio/Makefile	Thu Jan 17 12:28:23 2008 +1100
@@ -1,3 +1,4 @@ obj-$(CONFIG_VIRTIO) += virtio.o
 obj-$(CONFIG_VIRTIO) += virtio.o
 obj-$(CONFIG_VIRTIO_RING) += virtio_ring.o
 obj-$(CONFIG_VIRTIO_PCI) += virtio_pci.o
+obj-$(CONFIG_VIRTIO_BALLOON) += virtio_balloon.o
diff -r c4762959de25 drivers/virtio/virtio_balloon.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/drivers/virtio/virtio_balloon.c	Thu Jan 17 12:28:23 2008 +1100
@@ -0,0 +1,235 @@
+/* Virtio balloon implementation, inspired by Dor Loar and Marcelo
+ * Tosatti's implementations.
+ *
+ *  Copyright 2008 Rusty Russell IBM Corporation
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+#define DEBUG
+#include <linux/virtio.h>
+#include <linux/virtio_balloon.h>
+#include <linux/swap.h>
+#include <linux/kthread.h>
+#include <linux/freezer.h>
+
+struct virtio_balloon
+{
+	struct virtio_device *vdev;
+	struct virtqueue *vq;
+
+	/* Where the ballooning thread waits for config to change. */
+	wait_queue_head_t config_change;
+
+	/* The thread servicing the balloon. */
+	struct task_struct *thread;
+
+	/* Waiting for host to ack the pages we released. */
+	struct completion acked;
+
+	/* The pages we've told the Host we're not using. */
+	unsigned int num_pages;
+	struct list_head pages;
+
+	/* The array of pfns we tell the Host about. */
+	unsigned int num_pfns;
+	u32 pfns[256];
+};
+
+static struct virtio_device_id id_table[] = {
+	{ VIRTIO_ID_BALLOON, VIRTIO_DEV_ANY_ID},
+	{ 0 },
+};
+
+static void leak_balloon(struct virtio_balloon *vb, unsigned int num)
+{
+	struct page *page;
+	unsigned int i;
+
+	/* Simply free pages, and usage will fault them back in. */
+	for (i = 0; i < num; i++) {
+		page = list_first_entry(&vb->pages, struct page, lru);
+		list_del(&page->lru);
+		__free_page(page);
+		vb->num_pages--;
+		totalram_pages++;
+	}
+}
+
+static void balloon_ack(struct virtqueue *vq)
+{
+	struct virtio_balloon *vb;
+	unsigned int len;
+
+	vb = vq->vq_ops->get_buf(vq, &len);
+	if (vb)
+		complete(&vb->acked);
+}
+
+static void fill_balloon(struct virtio_balloon *vb, unsigned int num)
+{
+	struct scatterlist sg;
+
+	/* We can only do one array worth at a time. */
+	num = min(num, ARRAY_SIZE(vb->pfns));
+
+	for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
+		struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY);
+		if (!page) {
+			if (printk_ratelimit())
+				dev_printk(KERN_INFO, &vb->vdev->dev,
+					   "Out of puff! Can't get %u pages\n",
+					   num);
+			/* Sleep for at least 1/5 of a second before retry. */
+			msleep(200);
+			break;
+		}
+		vb->pfns[vb->num_pfns] = page_to_pfn(page);
+		totalram_pages--;
+		vb->num_pages++;
+		list_add(&page->lru, &vb->pages);
+	}
+
+	/* Didn't get any?  Oh well. */
+	if (vb->num_pfns == 0)
+		return;
+	
+	init_completion(&vb->acked);
+	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
+
+	/* We should always be able to add one buffer to an empty queue. */
+	if (vb->vq->vq_ops->add_buf(vb->vq, &sg, 1, 0, vb) != 0)
+		BUG();
+	vb->vq->vq_ops->kick(vb->vq);
+
+	/* When host has read buffer, this completes via balloon_ack */
+	wait_for_completion(&vb->acked);
+}
+
+static void virtballoon_changed(struct virtio_device *vdev)
+{
+	struct virtio_balloon *vb = vdev->priv;
+
+	wake_up(&vb->config_change);
+}
+
+static inline int towards_target(struct virtio_balloon *vb)
+{
+	u32 v;
+	__virtio_config_val(vb->vdev,
+			    offsetof(struct virtio_balloon_config, num_pages),
+			    &v);
+	return v - vb->num_pages;
+}
+
+static int balloon(void *_vballoon)
+{
+	struct virtio_balloon *vb = _vballoon;
+
+	set_freezable();
+	while (!kthread_should_stop()) {
+		int diff;
+
+		try_to_freeze();
+		wait_event_interruptible(vb->config_change,
+					 (diff = towards_target(vb)) != 0
+					 || kthread_should_stop());
+		if (diff > 0)
+			fill_balloon(vb, diff);
+		else if (diff < 0)
+			leak_balloon(vb, -diff);
+	}
+	return 0;
+}
+
+static int virtballoon_probe(struct virtio_device *vdev)
+{
+	struct virtio_balloon *vb;
+	int err;
+
+	vdev->priv = vb = kmalloc(sizeof(*vb), GFP_KERNEL);
+	if (!vb) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	INIT_LIST_HEAD(&vb->pages);
+	vb->num_pages = 0;
+	init_waitqueue_head(&vb->config_change);
+	vb->vdev = vdev;
+
+	/* We expect one virtqueue, for output. */
+	vb->vq = vdev->config->find_vq(vdev, 0, balloon_ack);
+	if (IS_ERR(vb->vq)) {
+		err = PTR_ERR(vb->vq);
+		goto out_free_vb;
+	}
+
+	vb->thread = kthread_run(balloon, vb, "vballoon");
+	if (IS_ERR(vb->thread)) {
+		err = PTR_ERR(vb->thread);
+		goto out_del_vq;
+	}
+	return 0;
+
+out_del_vq:
+	vdev->config->del_vq(vb->vq);
+out_free_vb:
+	kfree(vb);
+out:
+	return err;
+}
+
+static void virtballoon_remove(struct virtio_device *vdev)
+{
+	struct virtio_balloon *vb = vdev->priv;
+	struct page *i, *next;
+
+	kthread_stop(vb->thread);
+
+	/* There might be pages left in the balloon: free them. */
+	list_for_each_entry_safe(i, next, &vb->pages, lru) {
+		vb->num_pages--;
+		__free_page(i);
+	}
+	BUG_ON(vb->num_pages);
+
+	vdev->config->del_vq(vb->vq);
+	kfree(vb);
+}
+
+static struct virtio_driver virtio_balloon = {
+	.driver.name =	KBUILD_MODNAME,
+	.driver.owner =	THIS_MODULE,
+	.id_table =	id_table,
+	.probe =	virtballoon_probe,
+	.remove =	__devexit_p(virtballoon_remove),
+	.config_changed = virtballoon_changed,
+};
+
+static int __init init(void)
+{
+	return register_virtio_driver(&virtio_balloon);
+}
+
+static void __exit fini(void)
+{
+	unregister_virtio_driver(&virtio_balloon);
+}
+module_init(init);
+module_exit(fini);
+
+MODULE_DEVICE_TABLE(virtio, id_table);
+MODULE_DESCRIPTION("Virtio balloon driver");
+MODULE_LICENSE("GPL");
diff -r c4762959de25 include/linux/virtio_balloon.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/include/linux/virtio_balloon.h	Thu Jan 17 12:28:23 2008 +1100
@@ -0,0 +1,13 @@
+#ifndef _LINUX_VIRTIO_BALLOON_H
+#define _LINUX_VIRTIO_BALLOON_H
+#include <linux/virtio_config.h>
+
+/* The ID for virtio_balloon */
+#define VIRTIO_ID_BALLOON	5
+
+struct virtio_balloon_config
+{
+	/* Number of pages host wants Guest to give up. */
+	__le32 num_pages;
+};
+#endif /* _LINUX_VIRTIO_BALLOON_H */

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