Date:   Wed, 22 Jun 2022 16:35:52 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Ilya Maximets <i.maximets@....org>
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Florian Westphal <fw@...len.de>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, dev@...nvswitch.org,
        LKML <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] net: ensure all external references are released in
 deferred skbuffs

On Wed, Jun 22, 2022 at 4:26 PM Ilya Maximets <i.maximets@....org> wrote:
>
> On 6/22/22 13:43, Eric Dumazet wrote:
> > On Wed, Jun 22, 2022 at 1:32 PM Ilya Maximets <i.maximets@....org> wrote:
> >>
> >> On 6/22/22 12:36, Eric Dumazet wrote:
> >>> On Wed, Jun 22, 2022 at 12:28 PM Florian Westphal <fw@...len.de> wrote:
> >>>>
> >>>> Eric Dumazet <edumazet@...gle.com> wrote:
> >>>>> On Sun, Jun 19, 2022 at 2:39 AM Ilya Maximets <i.maximets@....org> wrote:
> >>>>>>
> >>>>>> Open vSwitch system test suite is broken due to inability to
> >>>>>> load/unload netfilter modules.  kworker thread is getting trapped
> >>>>>> in the infinite loop while running a net cleanup inside the
> >>>>>> nf_conntrack_cleanup_net_list, because deferred skbuffs are still
> >>>>>> holding nfct references and not being freed by their CPU cores.
> >>>>>>
> >>>>>> In general, the idea that we will have an rx interrupt on every
> >>>>>> CPU core at some point in a near future doesn't seem correct.
> >>>>>> Devices are getting created and destroyed, interrupts are getting
> >>>>>> re-scheduled, CPUs are going online and offline dynamically.
> >>>>>> Any of these events may leave packets stuck in defer list for a
> >>>>>> long time.  It might be OK, if they are just a piece of memory,
> >>>>>> but we can't afford them holding references to any other resources.
> >>>>>>
> >>>>>> In case of OVS, nfct reference keeps the kernel thread in busy loop
> >>>>>> while holding a 'pernet_ops_rwsem' semaphore.  That blocks the
> >>>>>> later modprobe request from user space:
> >>>>>>
> >>>>>>   # ps
> >>>>>>    299 root  R  99.3  200:25.89 kworker/u96:4+
> >>>>>>
> >>>>>>   # journalctl
> >>>>>>   INFO: task modprobe:11787 blocked for more than 1228 seconds.
> >>>>>>         Not tainted 5.19.0-rc2 #8
> >>>>>>   task:modprobe     state:D
> >>>>>>   Call Trace:
> >>>>>>    <TASK>
> >>>>>>    __schedule+0x8aa/0x21d0
> >>>>>>    schedule+0xcc/0x200
> >>>>>>    rwsem_down_write_slowpath+0x8e4/0x1580
> >>>>>>    down_write+0xfc/0x140
> >>>>>>    register_pernet_subsys+0x15/0x40
> >>>>>>    nf_nat_init+0xb6/0x1000 [nf_nat]
> >>>>>>    do_one_initcall+0xbb/0x410
> >>>>>>    do_init_module+0x1b4/0x640
> >>>>>>    load_module+0x4c1b/0x58d0
> >>>>>>    __do_sys_init_module+0x1d7/0x220
> >>>>>>    do_syscall_64+0x3a/0x80
> >>>>>>    entry_SYSCALL_64_after_hwframe+0x46/0xb0
> >>>>>>
> >>>>>> At this point OVS testsuite is unresponsive and never recover,
> >>>>>> because these skbuffs are never freed.
> >>>>>>
> >>>>>> Solution is to make sure no external references attached to skb
> >>>>>> before pushing it to the defer list.  Using skb_release_head_state()
> >>>>>> for that purpose.  The function modified to be re-enterable, as it
> >>>>>> will be called again during the defer list flush.
> >>>>>>
> >>>>>> Another approach that can fix the OVS use-case, is to kick all
> >>>>>> cores while waiting for references to be released during the net
> >>>>>> cleanup.  But that sounds more like a workaround for a current
> >>>>>> issue rather than a proper solution and will not cover possible
> >>>>>> issues in other parts of the code.
> >>>>>>
> >>>>>> Additionally checking for skb_zcopy() while deferring.  This might
> >>>>>> not be necessary, as I'm not sure if we can actually have zero copy
> >>>>>> packets on this path, but seems worth having for completeness as we
> >>>>>> should never defer such packets regardless.
> >>>>>>
> >>>>>> CC: Eric Dumazet <edumazet@...gle.com>
> >>>>>> Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
> >>>>>> Signed-off-by: Ilya Maximets <i.maximets@....org>
> >>>>>> ---
> >>>>>>  net/core/skbuff.c | 16 +++++++++++-----
> >>>>>>  1 file changed, 11 insertions(+), 5 deletions(-)
> >>>>>
> >>>>> I do not think this patch is doing the right thing.
> >>>>>
> >>>>> Packets sitting in TCP receive queues should not hold state that is
> >>>>> not relevant for TCP recvmsg().
> >>>>
> >>>> Agree, but tcp_v4/6_rcv() already call nf_reset_ct(), else it would
> >>>> not be possible to remove nf_conntrack module in practice.
> >>>
> >>> Well, existing nf_reset_ct() does not catch all cases, like TCP fastopen ?
> >>
> >> Yeah, that is kind of the main problem I have with the current
> >> code.  It's very hard to find all the cases where skb has to be
> >> cleaned up and almost impossible for someone who doesn't know
> >> every aspect of every network subsystem in the kernel.  That's
> >> why I went with the more or less bulletproof approach of cleaning
> >> up while actually deferring.  I can try and test the code you
> >> proposed in the other reply, but at least, I think, we need a
> >> bunch of debug warnings in the skb_attempt_defer_free() to catch
> >> possible issues.
> >
> > Debug warnings are expensive if they need to bring new cache lines.
> >
> > So far skb_attempt_defer_free() is only used by TCP in well known conditions.
>
> That's true for skb_attempt_defer_free() itself, but it's
> hard to tell the same for all the parts of the code that
> are enqueuing these skbuffs.  Even if all the places are
> well known, it looks to me highly error prone.  Having
> a couple of DEBUG_NET_WARN_ON_ONCE should not cause any
> performance issues, IIUC, unless debug is enabled, right?
>
> e.g.
>
>         DEBUG_NET_WARN_ON_ONCE(skb_nfct(skb));
>         DEBUG_NET_WARN_ON_ONCE(skb_dst(skb));

Sure, but we need to put them at the right place.

Doing these checks in skb_attempt_defer_free() is too late.

>
> >
> >
> >>
> >> Also, what about cleaning up extensions?  IIUC, at least one
> >> of them can hold external references.  SKB_EXT_SEC_PATH holds
> >> xfrm_state.  We should, probably, free them as well?
> >
> > I do not know about this, I would ask XFRM maintainers
>
> @Steffen, @Herbert, what do you think?  Is it a problem if the
> skb holding SKB_EXT_SEC_PATH extension is held not freed in
> a defer list indefinitely (for a very long time)?  Or is it
> even possible for an skb with SKB_EXT_SEC_PATH extension present
> to be enqueued to a TCP receive queue without releasing all
> the references?
>
> This seems like a long existing problem though, so can be fixed
> separately, if it is a problem.
>
> >
> >>
> >> And what about zerocopy skb?  I think, we should still not
> >> allow them to be deferred as they seem to hold HW resources.
> >
> > The point of skb_attempt_defer_free() i is to make the freeing happen
> > at producer
> >  much instead of consumer.
> >
> > I do not think there is anything special in this regard with zero
> > copy. I would leave the current code as is.
>
> The problem is that these skbuffs are never actually freed,
> so if they do hold HW resources, these resources will never be
> released (never == for a very long time, hours, days).

This is fine. Number of such skbs per cpu is limited.

Or if some HW feature needs buffers to be released _immediately_, we
have to implement
a method for doing so.

Please point why buffers sitting in TCP receive queues are not causing issues ?
Do we need a general method "Please scan all TCP sockets and release
skb holding HW resources immediately"

>
> Not a blocker from my point of view, just trying to highlight
> a possible issue.

The tradeoff is known really, as for any cache.

>
> >
> > A simpler patch might be to move the existing nf_reset_ct() earlier,
> > can you test this ?
>
> I tested the patch below and it seems to fix the issue seen
> with OVS testsuite.  Though it's not obvious for me why this
> happens.  Can you explain a bit more?

I think the bug predates my recent change.

skb_attempt_defer_free() is increasing the probability of hitting the bug.

>
> >
> > I note that IPv6 does the nf_reset_ct() earlier, from ip6_protocol_deliver_rcu()
> >
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index fda811a5251f2d76ac24a036e6b4f4e7d7d96d6f..a06464f96fe0cc94dd78272738ddaab2c19e87db
> > 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -1919,6 +1919,8 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >         struct sock *sk;
> >         int ret;
> >
> > +       nf_reset_ct(skb);
> > +
> >         drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
> >         if (skb->pkt_type != PACKET_HOST)
> >                 goto discard_it;
> > @@ -2046,8 +2048,6 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >         if (drop_reason)
> >                 goto discard_and_relse;
> >
> > -       nf_reset_ct(skb);
> > -
> >         if (tcp_filter(sk, skb)) {
> >                 drop_reason = SKB_DROP_REASON_SOCKET_FILTER;
> >                 goto discard_and_relse;
>

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