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Date: Fri, 8 Jan 2016 14:56:14 -0500
From: Rafael Aquini <aquini@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Minchan Kim <minchan@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
Konstantin Khlebnikov <koct9i@...il.com>,
stable@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio_balloon: fix race between migration and
ballooning
On Fri, Jan 01, 2016 at 11:36:13AM +0200, Michael S. Tsirkin wrote:
> 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@...r.kernel.org>
> > Signed-off-by: Minchan Kim <minchan@...nel.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.
>
Nice catch! Thanks for spotting it. I just have one minor nit. See
below
>
> 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;
^^^^[1]
> }
> + put_page(page);
> + spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> }
>
> + /* re-add remaining entries */
> + list_splice(&processed, &b_dev_info->pages);
By breaking the loop at its ordinary and expected way-out case [1]
we'll hit list_splice without holding b_dev_info->pages_lock, won't we?
perhaps by adding the following on top of your patch we can address that pickle
aforementioned:
Cheers!
Rafael
--
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 66d69c5..74b3e9c 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -58,7 +58,7 @@ struct page *balloon_page_dequeue(struct
balloon_dev_info *b_dev_info)
{
struct page *page;
unsigned long flags;
- bool dequeued_page;
+ bool dequeued_page, locked;
LIST_HEAD(processed); /* protected by b_dev_info->pages_lock */
dequeued_page = false;
@@ -105,13 +105,17 @@ struct page *balloon_page_dequeue(struct
balloon_dev_info *b_dev_info)
unlock_page(page);
put_page(page);
dequeued_page = true;
+ locked = false;
break;
}
put_page(page);
spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+ locked = true;
}
/* re-add remaining entries */
+ if (!locked)
+ spin_lock_irqsave(&b_dev_info->pages_lock, flags);
list_splice(&processed, &b_dev_info->pages);
spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
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