[RFC] virtio_balloon: disable oom killer when fill balloon

[Dave Young <hidave.darkstar@gmail.com>]

Balloon could cause guest memory oom killing and panic. If we disable the oom killer it will be better at least avoid guest panic.

If alloc failed we can just adjust the balloon target to be equal to current number by call vdev->config->set

But during test I found the config->set num_pages does not change the config actually, Should I do hacks in userspace as well? If so where should I start to hack? 

Thanks

--- linux-2.6.orig/drivers/virtio/virtio_balloon.c	2010-09-25 20:58:14.190000001 +0800
+++ linux-2.6/drivers/virtio/virtio_balloon.c	2010-09-28 21:05:42.203333675 +0800
@@ -25,6 +25,7 @@
 #include <linux/freezer.h>
 #include <linux/delay.h>
 #include <linux/slab.h>
+#include <linux/oom.h>
 
 struct virtio_balloon
 {
@@ -97,8 +98,22 @@ static void tell_host(struct virtio_ball
 	wait_for_completion(&vb->acked);
 }
 
-static void fill_balloon(struct virtio_balloon *vb, size_t num)
+static int cblimit(int times)
 {
+	static int t;
+
+	if (t < times)
+		t++;
+	else
+		t = 0;
+
+	return !t;
+}
+
+static int fill_balloon(struct virtio_balloon *vb, size_t num)
+{
+	int ret = 0;
+
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
@@ -106,10 +121,13 @@ static void fill_balloon(struct virtio_b
 		struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
 					__GFP_NOMEMALLOC | __GFP_NOWARN);
 		if (!page) {
-			if (printk_ratelimit())
+			if (cblimit(5)) {
 				dev_printk(KERN_INFO, &vb->vdev->dev,
 					   "Out of puff! Can't get %zu pages\n",
 					   num);
+				ret = -ENOMEM;
+				goto out;
+			}
 			/* Sleep for at least 1/5 of a second before retry. */
 			msleep(200);
 			break;
@@ -120,11 +138,11 @@ static void fill_balloon(struct virtio_b
 		list_add(&page->lru, &vb->pages);
 	}
 
-	/* Didn't get any?  Oh well. */
-	if (vb->num_pfns == 0)
-		return;
+out:
+	if (vb->num_pfns)
+		tell_host(vb, vb->inflate_vq);
 
-	tell_host(vb, vb->inflate_vq);
+	return ret;
 }
 
 static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
@@ -251,6 +269,14 @@ static void update_balloon_size(struct v
 			      &actual, sizeof(actual));
 }
 
+static void update_balloon_target(struct virtio_balloon *vb)
+{
+	__le32 num_pages = cpu_to_le32(vb->num_pages);
+	vb->vdev->config->set(vb->vdev,
+			      offsetof(struct virtio_balloon_config, num_pages),
+			      &num_pages, sizeof(num_pages));
+}
+
 static int balloon(void *_vballoon)
 {
 	struct virtio_balloon *vb = _vballoon;
@@ -267,9 +293,14 @@ static int balloon(void *_vballoon)
 					 || freezing(current));
 		if (vb->need_stats_update)
 			stats_handle_request(vb);
-		if (diff > 0)
-			fill_balloon(vb, diff);
-		else if (diff < 0)
+		if (diff > 0) {
+			int oom;
+			oom_killer_disable();
+			oom = fill_balloon(vb, diff);
+			oom_killer_enable();
+			if (oom)
+				update_balloon_target(vb);
+		} else if (diff < 0)
 			leak_balloon(vb, -diff);
 		update_balloon_size(vb);
 	}