netdev - Re: nf-nat-core: allocated memory at module unload.

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Message-ID: <CAJuCfpEDKkiXm1ye=gs3ohLDJM7gqQc0WwS=6egddbsZ1qRF9A@mail.gmail.com>
Date: Mon, 7 Oct 2024 08:10:22 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: Ben Greear <greearb@...delatech.com>, netdev <netdev@...r.kernel.org>, 
	kent.overstreet@...ux.dev, pablo@...filter.org
Subject: Re: nf-nat-core: allocated memory at module unload.

On Mon, Oct 7, 2024 at 4:29 AM Florian Westphal <fw@...len.de> wrote:
>
> Suren Baghdasaryan <surenb@...gle.com> wrote:
> > On Tue, Oct 1, 2024 at 12:36 PM Florian Westphal <fw@...len.de> wrote:
> > >
> > > Ben Greear <greearb@...delatech.com> wrote:
> > >
> > > [ CCing codetag folks ]
> >
> > Thanks! I've been on vacation and just saw this report.
> >
> > >
> > > > Hello,
> > > >
> > > > I see this splat in 6.11.0 (plus a single patch to fix vrf xmit deadlock).
> > > >
> > > > Is this a known issue?  Is it a serious problem?
> > >
> > > Not known to me.  Looks like an mm (rcu)+codetag problem.
> > >
> > > > ------------[ cut here ]------------
> > > > net/netfilter/nf_nat_core.c:1114 module nf_nat func:nf_nat_register_fn has 256 allocated at module unload
> > > > WARNING: CPU: 1 PID: 10421 at lib/alloc_tag.c:168 alloc_tag_module_unload+0x22b/0x3f0
> > > > Modules linked in: nf_nat(-) btrfs ufs qnx4 hfsplus hfs minix vfat msdos fat
> > > ...
> > > > Hardware name: Default string Default string/SKYBAY, BIOS 5.12 08/04/2020
> > > > RIP: 0010:alloc_tag_module_unload+0x22b/0x3f0
> > > >  codetag_unload_module+0x19b/0x2a0
> > > >  ? codetag_load_module+0x80/0x80
> > > >  ? up_write+0x4f0/0x4f0
> > >
> > > "Well, yes, but actually no."
> > >
> > > At this time, kfree_rcu() has been called on all 4 objects.
> > >
> > > Looks like kfree_rcu no longer cares even about rcu_barrier(), and
> > > there is no kvfree_rcu_barrier() in 6.11.
> > >
> > > The warning goes away when I replace kfree_rcu with call_rcu+kfree
> > > plus rcu_barrier in module exit path.
> > >
> > > But I don't think its the right thing to do.
> > >
> > > (referring to nf_nat_unregister_fn(), kfree_rcu(priv, rcu_head);).
> > >
> > > Reproducer:
> > > unshare -n iptables-nft -t nat -A PREROUTING -p tcp
> > > grep nf_nat /proc/allocinfo # will list 4 allocations
> > > rmmod nft_chain_nat
> > > rmmod nf_nat                # will WARN.
> > >
> > > Without rmmod, the 4 allocations go away after a few seconds,
> > > grep will no longer list them and then rmmod won't splat.
> >
> > I see. So, the kfree_rcu() was already called but freeing did not
> > happen yet, in the meantime we are unloading the module.
>
> Yes.
>
> > We could add
> > a synchronize_rcu() at the beginning of codetag_unload_module() so
> > that all pending kfree_rcu()s complete before we check codetag
> > counters:
> >
> > bool codetag_unload_module(struct module *mod)
> > {
> >         struct codetag_type *cttype;
> >         bool unload_ok = true;
> >
> >         if (!mod)
> >                 return true;
> >
> > +      synchronize_rcu();
> >         mutex_lock(&codetag_lock);
>
> This doesn't help as kfree_rcu doesn't wait for this.
>
> Use of kvfree_rcu_barrier() instead does work though.

I see. That sounds like an acceptable fix. Please post it and I'll ack it.
Thanks!