Re: KVM: memory ballooning bug?

[Rafael Aquini <aquini@redhat.com>]

On Wed, Dec 23, 2015 at 02:22:28PM +0900, Minchan Kim wrote:
> During my compaction-related stuff, I encountered some problems with
> ballooning.
> 
> Firstly, with repeated inflating and deflating cycle, guest memory(ie,
> cat /proc/meminfo | grep MemTotal) decreased and couldn't recover.
> 
> When I review source code, balloon_lock should cover release_pages_balloon.
> Otherwise, struct virtio_balloon fields could be overwritten by race
> of fill_balloon(e,g, vb->*pfns could be critical).
> Below patch fixed the problem.
> 
> 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;
>  }
>  
> Secondly, in balloon_page_dequeue, pages_lock should cover
> list_for_each_entry_safe loop. Otherwise, the cursor page
> could be isolated by compaction and then list_del by isolation
> could poison the page->lru so the loop could access wrong address
> like this.
> 
> 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+ #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
> 
> We could fix it by protecting the entire loop by pages_lock but
> problem is irq latency during walking the list.
> But I doubt how often such worst scenario happens because
> in normal situation, the loop would exit easily via succeeding
> trylock_page.
> 
> Any comments?

Nope, I think the simplest way to address both cases you stumbled
across is by replacing the locking to extend those critical sections as
you suggested.

Merry Xmas!
-- Rafael

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