Re: KVM: memory ballooning bug?

From: Rafael Aquini
Date: Wed Dec 23 2015 - 10:14:11 EST

On Wed, Dec 23, 2015 at 11:17:10PM +0900, Minchan Kim wrote:
> On Wed, Dec 23, 2015 at 06:14:49AM -0500, Rafael Aquini wrote:
> > 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.
>
> I couldn't understand why you said "Nope" and which lock do you mean?
> There are two locks to need to extend.
> If you are on same page with me, I suggested this.
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 7efc329..7d3e5d0 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;
> }
>
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index d3116be..300117f 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) {
> /*
> Do you agree this patch? If so, I will send patch after X-mas.

That's precisely what I was thinking :)

Happy Xmas!
-- Rafael
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