thr3ads.net - Linux Virtualization - [PATCH 1/2] virtio_balloon: fix race by fill and leak [Dec 2015]

If this information is useful, please help other people find it:
Share via:

Minchan Kim

2015-Dec-27 23:35 UTC

[PATCH 1/2] virtio_balloon: fix race by fill and leak

During my compaction-related stuff, I encountered a bug
with ballooning.

With repeated inflating and deflating cycle, guest memory(
ie, cat /proc/meminfo | grep MemTotal) is decreased and
couldn't be recovered.

The reason is balloon_lock doesn't cover release_pages_balloon
so struct virtio_balloon fields could be overwritten by race
of fill_balloon(e,g, vb->*pfns could be critical).

This patch fixes it in my test.

Cc: <stable at vger.kernel.org>
Signed-off-by: Minchan Kim <minchan at kernel.org>
---
 drivers/virtio/virtio_balloon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 7efc32945810..7d3e5d0e9aa4 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -209,8 +209,8 @@ 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_balloon(vb);
+	mutex_unlock(&vb->balloon_lock);
 	return num_freed_pages;
 }
 
-- 
1.9.1

Minchan Kim

2015-Dec-27 23:35 UTC

head link

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

In balloon_page_dequeue, pages_lock should cover the loop
(ie, list_for_each_entry_safe). Otherwise, the cursor page could
be isolated by compaction and then list_del by isolation could
poison the page->lru.{prev,next} so the loop finally could
access wrong address like this. This patch fixes the bug.

general protection fault: 0000 [#1] SMP
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 2 PID: 82 Comm: vballoon Not tainted 4.4.0-rc5-mm1-access_bit+ #1906
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff8800a7ff0000 ti: ffff8800a7fec000 task.ti: ffff8800a7fec000
RIP: 0010:[<ffffffff8115e754>]  [<ffffffff8115e754>]
balloon_page_dequeue+0x54/0x130
RSP: 0018:ffff8800a7fefdc0  EFLAGS: 00010246
RAX: ffff88013fff9a70 RBX: ffffea000056fe00 RCX: 0000000000002b7d
RDX: ffff88013fff9a70 RSI: ffffea000056fe00 RDI: ffff88013fff9a68
RBP: ffff8800a7fefde8 R08: ffffea000056fda0 R09: 0000000000000000
R10: ffff8800a7fefd90 R11: 0000000000000001 R12: dead0000000000e0
R13: ffffea000056fe20 R14: ffff880138809070 R15: ffff880138809060
FS:  0000000000000000(0000) GS:ffff88013fc40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f229c10e000 CR3: 00000000b8b53000 CR4: 00000000000006a0
Stack:
 0000000000000100 ffff880138809088 ffff880138809000 ffff880138809060
 0000000000000046 ffff8800a7fefe28 ffffffff812c86d3 ffff880138809020
 ffff880138809000 fffffffffff91900 0000000000000100 ffff880138809060
Call Trace:
 [<ffffffff812c86d3>] leak_balloon+0x93/0x1a0
 [<ffffffff812c8bc7>] balloon+0x217/0x2a0
 [<ffffffff8143739e>] ? __schedule+0x31e/0x8b0
 [<ffffffff81078160>] ? abort_exclusive_wait+0xb0/0xb0
 [<ffffffff812c89b0>] ? update_balloon_stats+0xf0/0xf0
 [<ffffffff8105b6e9>] kthread+0xc9/0xe0
 [<ffffffff8105b620>] ? kthread_park+0x60/0x60
 [<ffffffff8143b4af>] ret_from_fork+0x3f/0x70
 [<ffffffff8105b620>] ? kthread_park+0x60/0x60
Code: 8d 60 e0 0f 84 af 00 00 00 48 8b 43 20 a8 01 75 3b 48 89 d8 f0 0f ba 28 00
72 10 48 8b 03 f6 c4 08 75 2f 48 89 df e8 8c 83 f9 ff <49> 8b 44 24 20 4d
8d 6c 24 20 48 83 e8 20 4d 39 f5 74 7a 4c 89
RIP  [<ffffffff8115e754>] balloon_page_dequeue+0x54/0x130
 RSP <ffff8800a7fefdc0>
---[ end trace 43cf28060d708d5f ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled

Cc: <stable at vger.kernel.org>
Signed-off-by: Minchan Kim <minchan at kernel.org>
---
 mm/balloon_compaction.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index d3116be5a00f..300117f1a08f 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -61,6 +61,7 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
 	bool dequeued_page;
 
 	dequeued_page = false;
+	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
 	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
 		/*
 		 * Block others from accessing the 'page' while we get around
@@ -75,15 +76,14 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
 				continue;
 			}
 #endif
-			spin_lock_irqsave(&b_dev_info->pages_lock, flags);
 			balloon_page_delete(page);
 			__count_vm_event(BALLOON_DEFLATE);
-			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
 			unlock_page(page);
 			dequeued_page = true;
 			break;
 		}
 	}
+	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
 
 	if (!dequeued_page) {
 		/*
-- 
1.9.1

Rafael Aquini

2015-Dec-27 23:36 UTC

head link

[PATCH 1/2] virtio_balloon: fix race by fill and leak

On Mon, Dec 28, 2015 at 08:35:12AM +0900, Minchan Kim
wrote:> During my compaction-related stuff, I encountered a bug
> with ballooning.
> 
> With repeated inflating and deflating cycle, guest memory(
> ie, cat /proc/meminfo | grep MemTotal) is decreased and
> couldn't be recovered.
> 
> The reason is balloon_lock doesn't cover release_pages_balloon
> so struct virtio_balloon fields could be overwritten by race
> of fill_balloon(e,g, vb->*pfns could be critical).
> 
> This patch fixes it in my test.
> 
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Minchan Kim <minchan at kernel.org>
> ---
>  drivers/virtio/virtio_balloon.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index 7efc32945810..7d3e5d0e9aa4 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -209,8 +209,8 @@ 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_balloon(vb);
> +	mutex_unlock(&vb->balloon_lock);
>  	return num_freed_pages;
>  }
>  
> -- 
> 1.9.1
> Acked-by: Rafael Aquini <aquini at redhat.com>

Rafael Aquini

2015-Dec-27 23:36 UTC

head link

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

On Mon, Dec 28, 2015 at 08:35:13AM +0900, Minchan Kim
wrote:> In balloon_page_dequeue, pages_lock should cover the loop
> (ie, list_for_each_entry_safe). Otherwise, the cursor page could
> be isolated by compaction and then list_del by isolation could
> poison the page->lru.{prev,next} so the loop finally could
> access wrong address like this. This patch fixes the bug.
> 
> general protection fault: 0000 [#1] SMP
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Modules linked in:
> CPU: 2 PID: 82 Comm: vballoon Not tainted 4.4.0-rc5-mm1-access_bit+ #1906
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
01/01/2011
> task: ffff8800a7ff0000 ti: ffff8800a7fec000 task.ti: ffff8800a7fec000
> RIP: 0010:[<ffffffff8115e754>]  [<ffffffff8115e754>]
balloon_page_dequeue+0x54/0x130
> RSP: 0018:ffff8800a7fefdc0  EFLAGS: 00010246
> RAX: ffff88013fff9a70 RBX: ffffea000056fe00 RCX: 0000000000002b7d
> RDX: ffff88013fff9a70 RSI: ffffea000056fe00 RDI: ffff88013fff9a68
> RBP: ffff8800a7fefde8 R08: ffffea000056fda0 R09: 0000000000000000
> R10: ffff8800a7fefd90 R11: 0000000000000001 R12: dead0000000000e0
> R13: ffffea000056fe20 R14: ffff880138809070 R15: ffff880138809060
> FS:  0000000000000000(0000) GS:ffff88013fc40000(0000)
knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 00007f229c10e000 CR3: 00000000b8b53000 CR4: 00000000000006a0
> Stack:
>  0000000000000100 ffff880138809088 ffff880138809000 ffff880138809060
>  0000000000000046 ffff8800a7fefe28 ffffffff812c86d3 ffff880138809020
>  ffff880138809000 fffffffffff91900 0000000000000100 ffff880138809060
> Call Trace:
>  [<ffffffff812c86d3>] leak_balloon+0x93/0x1a0
>  [<ffffffff812c8bc7>] balloon+0x217/0x2a0
>  [<ffffffff8143739e>] ? __schedule+0x31e/0x8b0
>  [<ffffffff81078160>] ? abort_exclusive_wait+0xb0/0xb0
>  [<ffffffff812c89b0>] ? update_balloon_stats+0xf0/0xf0
>  [<ffffffff8105b6e9>] kthread+0xc9/0xe0
>  [<ffffffff8105b620>] ? kthread_park+0x60/0x60
>  [<ffffffff8143b4af>] ret_from_fork+0x3f/0x70
>  [<ffffffff8105b620>] ? kthread_park+0x60/0x60
> Code: 8d 60 e0 0f 84 af 00 00 00 48 8b 43 20 a8 01 75 3b 48 89 d8 f0 0f ba
28 00 72 10 48 8b 03 f6 c4 08 75 2f 48 89 df e8 8c 83 f9 ff <49> 8b 44 24
20 4d 8d 6c 24 20 48 83 e8 20 4d 39 f5 74 7a 4c 89
> RIP  [<ffffffff8115e754>] balloon_page_dequeue+0x54/0x130
>  RSP <ffff8800a7fefdc0>
> ---[ end trace 43cf28060d708d5f ]---
> Kernel panic - not syncing: Fatal exception
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled
> 
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Minchan Kim <minchan at kernel.org>
> ---
>  mm/balloon_compaction.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index d3116be5a00f..300117f1a08f 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -61,6 +61,7 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
>  	bool dequeued_page;
>  
>  	dequeued_page = false;
> +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
>  		/*
>  		 * Block others from accessing the 'page' while we get around
> @@ -75,15 +76,14 @@ struct page *balloon_page_dequeue(struct
balloon_dev_info *b_dev_info)
>  				continue;
>  			}
>  #endif
> -			spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  			balloon_page_delete(page);
>  			__count_vm_event(BALLOON_DEFLATE);
> -			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>  			unlock_page(page);
>  			dequeued_page = true;
>  			break;
>  		}
>  	}
> +	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>  
>  	if (!dequeued_page) {
>  		/*
> -- 
> 1.9.1
> Acked-by: Rafael Aquini <aquini at redhat.com>

Michael S. Tsirkin

2016-Jan-01 08:26 UTC

head link

[PATCH 1/2] virtio_balloon: fix race by fill and leak

On Mon, Dec 28, 2015 at 08:35:12AM +0900, Minchan Kim
wrote:> During my compaction-related stuff, I encountered a bug
> with ballooning.
> 
> With repeated inflating and deflating cycle, guest memory(
> ie, cat /proc/meminfo | grep MemTotal) is decreased and
> couldn't be recovered.
> 
> The reason is balloon_lock doesn't cover release_pages_balloon
> so struct virtio_balloon fields could be overwritten by race
> of fill_balloon(e,g, vb->*pfns could be critical).
> 
> This patch fixes it in my test.
> 
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Minchan Kim <minchan at kernel.org>
Acked-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index 7efc32945810..7d3e5d0e9aa4 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -209,8 +209,8 @@ 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_balloon(vb);
> +	mutex_unlock(&vb->balloon_lock);
>  	return num_freed_pages;
>  }
>  
> -- 
> 1.9.1

Michael S. Tsirkin

2016-Jan-01 09:36 UTC

head link

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

On Mon, Dec 28, 2015 at 08:35:13AM +0900, Minchan Kim
wrote:> In balloon_page_dequeue, pages_lock should cover the loop
> (ie, list_for_each_entry_safe). Otherwise, the cursor page could
> be isolated by compaction and then list_del by isolation could
> poison the page->lru.{prev,next} so the loop finally could
> access wrong address like this. This patch fixes the bug.
> 
> general protection fault: 0000 [#1] SMP
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Modules linked in:
> CPU: 2 PID: 82 Comm: vballoon Not tainted 4.4.0-rc5-mm1-access_bit+ #1906
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
01/01/2011
> task: ffff8800a7ff0000 ti: ffff8800a7fec000 task.ti: ffff8800a7fec000
> RIP: 0010:[<ffffffff8115e754>]  [<ffffffff8115e754>]
balloon_page_dequeue+0x54/0x130
> RSP: 0018:ffff8800a7fefdc0  EFLAGS: 00010246
> RAX: ffff88013fff9a70 RBX: ffffea000056fe00 RCX: 0000000000002b7d
> RDX: ffff88013fff9a70 RSI: ffffea000056fe00 RDI: ffff88013fff9a68
> RBP: ffff8800a7fefde8 R08: ffffea000056fda0 R09: 0000000000000000
> R10: ffff8800a7fefd90 R11: 0000000000000001 R12: dead0000000000e0
> R13: ffffea000056fe20 R14: ffff880138809070 R15: ffff880138809060
> FS:  0000000000000000(0000) GS:ffff88013fc40000(0000)
knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 00007f229c10e000 CR3: 00000000b8b53000 CR4: 00000000000006a0
> Stack:
>  0000000000000100 ffff880138809088 ffff880138809000 ffff880138809060
>  0000000000000046 ffff8800a7fefe28 ffffffff812c86d3 ffff880138809020
>  ffff880138809000 fffffffffff91900 0000000000000100 ffff880138809060
> Call Trace:
>  [<ffffffff812c86d3>] leak_balloon+0x93/0x1a0
>  [<ffffffff812c8bc7>] balloon+0x217/0x2a0
>  [<ffffffff8143739e>] ? __schedule+0x31e/0x8b0
>  [<ffffffff81078160>] ? abort_exclusive_wait+0xb0/0xb0
>  [<ffffffff812c89b0>] ? update_balloon_stats+0xf0/0xf0
>  [<ffffffff8105b6e9>] kthread+0xc9/0xe0
>  [<ffffffff8105b620>] ? kthread_park+0x60/0x60
>  [<ffffffff8143b4af>] ret_from_fork+0x3f/0x70
>  [<ffffffff8105b620>] ? kthread_park+0x60/0x60
> Code: 8d 60 e0 0f 84 af 00 00 00 48 8b 43 20 a8 01 75 3b 48 89 d8 f0 0f ba
28 00 72 10 48 8b 03 f6 c4 08 75 2f 48 89 df e8 8c 83 f9 ff <49> 8b 44 24
20 4d 8d 6c 24 20 48 83 e8 20 4d 39 f5 74 7a 4c 89
> RIP  [<ffffffff8115e754>] balloon_page_dequeue+0x54/0x130
>  RSP <ffff8800a7fefdc0>
> ---[ end trace 43cf28060d708d5f ]---
> Kernel panic - not syncing: Fatal exception
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled
> 
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Minchan Kim <minchan at kernel.org>
> ---
>  mm/balloon_compaction.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index d3116be5a00f..300117f1a08f 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -61,6 +61,7 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
>  	bool dequeued_page;
>  
>  	dequeued_page = false;
> +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
>  		/*
>  		 * Block others from accessing the 'page' while we get around
> @@ -75,15 +76,14 @@ struct page *balloon_page_dequeue(struct
balloon_dev_info *b_dev_info)
>  				continue;
>  			}
>  #endif
> -			spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  			balloon_page_delete(page);
>  			__count_vm_event(BALLOON_DEFLATE);
> -			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>  			unlock_page(page);
>  			dequeued_page = true;
>  			break;
>  		}
>  	}
> +	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>  
>  	if (!dequeued_page) {
>  		/*
I think this will cause deadlocks.

pages_lock now nests within page lock, balloon_page_putback
nests them in the reverse order.

Did you test this with lockdep? You really should for
locking changes, and I'd expect it to warn about this.

Also, there's another issue there I think: after isolation page could
also get freed before we try to lock it.

We really must take a page reference before touching
the page.

I think we need something like the below to fix this issue.
Could you please try this out, and send Tested-by?
I will repost as a proper patch if this works for you.


diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index d3116be..66d69c5 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -56,12 +56,34 @@ EXPORT_SYMBOL_GPL(balloon_page_enqueue);
  */
 struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
 {
-	struct page *page, *tmp;
+	struct page *page;
 	unsigned long flags;
 	bool dequeued_page;
+	LIST_HEAD(processed); /* protected by b_dev_info->pages_lock */
 
 	dequeued_page = false;
-	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
+	/*
+	 * We need to go over b_dev_info->pages and lock each page,
+	 * but b_dev_info->pages_lock must nest within page lock.
+	 *
+	 * To make this safe, remove each page from b_dev_info->pages list
+	 * under b_dev_info->pages_lock, then drop this lock. Once list is
+	 * empty, re-add them also under b_dev_info->pages_lock.
+	 */
+	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	while (!list_empty(&b_dev_info->pages)) {
+		page = list_first_entry(&b_dev_info->pages, typeof(*page), lru);
+		/* move to processed list to avoid going over it another time */
+		list_move(&page->lru, &processed);
+
+		if (!get_page_unless_zero(page))
+			continue;
+		/*
+		 * pages_lock nests within page lock,
+		 * so drop it before trylock_page
+		 */
+		spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+
 		/*
 		 * Block others from accessing the 'page' while we get around
 		 * establishing additional references and preparing the 'page'
@@ -72,6 +94,7 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
 			if (!PagePrivate(page)) {
 				/* raced with isolation */
 				unlock_page(page);
+				put_page(page);
 				continue;
 			}
 #endif
@@ -80,11 +103,18 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
 			__count_vm_event(BALLOON_DEFLATE);
 			spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
 			unlock_page(page);
+			put_page(page);
 			dequeued_page = true;
 			break;
 		}
+		put_page(page);
+		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
 	}
 
+	/* re-add remaining entries */
+	list_splice(&processed, &b_dev_info->pages);
+	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+
 	if (!dequeued_page) {
 		/*
 		 * If we are unable to dequeue a balloon page because the page

Apparently Analagous Threads

Search for more seemingly similar threads

Linux Virtualization - Dec 2015 - [PATCH 1/2] virtio_balloon: fix race by fill and leak

[PATCH 1/2] virtio_balloon: fix race by fill and leak

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

[PATCH 1/2] virtio_balloon: fix race by fill and leak

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

[PATCH 1/2] virtio_balloon: fix race by fill and leak

[PATCH 2/2] virtio_balloon: fix race between migration and ballooning

Apparently Analagous Threads