netdev - Re: Kernel memory leak in bnx2x driver with vxlan tunnel

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Date:	Tue, 19 Jan 2016 16:00:09 -0800
From:	Jesse Gross <jesse@...nel.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	John <john.phillips5@....com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Tom Herbert <tom@...bertland.com>, david.roth@....com,
	Pravin B Shelar <pshelar@...ira.com>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: Kernel memory leak in bnx2x driver with vxlan tunnel

On Tue, Jan 19, 2016 at 3:51 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2016-01-19 at 15:34 -0800, Jesse Gross wrote:
>> On Tue, Jan 19, 2016 at 2:47 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Tue, 2016-01-19 at 13:07 -0800, Jesse Gross wrote:
>> >> On Thu, Jan 14, 2016 at 9:17 AM, John <john.phillips5@....com> wrote:
>> >> > I'm getting what seems to be a kernel memory leak while doing a TCP
>> >> > throughput test between two VMs on identical systems, in order to test a
>> >> > broadcom NIC's performance with a kernel 4.4.0-rc8 and OpenVSwitch version
>> >> > 2.4.90. The host system of the receiving (server) VM leaks memory during the
>> >> > throughput test. The memory leaks fast enough to make the system completely
>> >> > unusable within five minutes. Once I stop the throughput test, the memory
>> >> > stops
>> >> > leaking. A couple of times, the kernel on the host system has actually
>> >> > killed
>> >> > the qemu process for me, but this doesn't happen reliably. The leaked memory
>> >> > doesn't become available again even after the VM is killed.
>> >>
>> >> It looks like the problem is in napi_skb_finish(). If we when do
>> >> GRO_MERGED_FREE we have NAPI_GRO_CB(skb)->free ==
>> >> NAPI_GRO_FREE_STOLEN_HEAD then we will just free the skb memory itself
>> >> but not any of the associated elements. Historically, this would have
>> >> been OK but these days we will have allocated a dst entry already for
>> >> tunnel metadata, which will get leaked.
>> >>
>> >> If we don't have NAPI_GRO_FREE_STOLEN_HEAD then we'll do a
>> >> __kfree_skb(), which will release the dst entry. That would explain
>> >> why some drivers have the problem but not others since the memory is
>> >> laid out differently.
>> >
>> >
>> > Wow.... What is the purpose of using skb_dst_set() on skb before calling
>> > gro_cells_receive() exactly ?
>> >
>> > Commit 2e15ea390e6f4466655066d97e22ec66870a042c changelog is not
>> > helpful :
>> >
>> >     Following patch create new tunnel flag which enable
>> >     tunnel metadata collection on given device.
>>
>> Note that this isn't really the problem commit. The general issue is
>> lightweight tunnels - the above commit is just adding support for GRE
>> in a way that I think follows the existing model.
>
> I believe this commit added the first skb_dst_set() before
> gro_cells_receive(), in ip_tunnel_rcv().
>
> This is already buggy.
>
> Then Tom in 58ce31cca1ffe057f4744c3f671e3e84606d3d4a added the
> gro_cells_receive() in vxlan_rcv(), which added another bug, because of
> existing skb_dst_set() call.

Oh, I wasn't disagreeing that there is a problem. I was just trying to
point out the conceptual root of the problem rather than the
individual instances in different tunnels implementations.

In any case, I think we can solve this by calling
skb_release_head_state() in napi_skb_finish() and switching early
demux to use skb_valid_dst().