netdev - Re: [PATCH v2 bpf-next 13/13] bpf: Convert bpf_cpumask to bpf_mem_cache_free

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Date: Mon, 26 Jun 2023 10:59:40 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: David Vernet <void@...ifault.com>
Cc: Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Hou Tao <houtao@...weicloud.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>, 
	rcu@...r.kernel.org, Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 13/13] bpf: Convert bpf_cpumask to bpf_mem_cache_free_rcu.

On Mon, Jun 26, 2023 at 10:55 AM David Vernet <void@...ifault.com> wrote:
>
> > > > +
> > > > +     migrate_disable();
> > > > +     bpf_mem_cache_free_rcu(&bpf_cpumask_ma, cpumask);
> > > > +     migrate_enable();
> > >
> > > The fact that callers have to disable migration like this in order to
> > > safely free the memory feels a bit leaky. Is there any reason we can't
> > > move this into bpf_mem_{cache_}free_rcu()?
> >
> > migrate_disable/enable() are actually not necessary here.
> > We can call bpf_mem_cache_free_rcu() directly from any kfunc.
>
> Could you please clarify why? Can't we migrate if the kfunc is called
> from a sleepable struct_ops callback?

migration is disabled for all bpf progs including sleepable.

> If migration is always disabled
> for any kfunc then I agree these migrate_{en,dis}able() calls can be
> removed. Otherwise from my reading of the code we'd race between calling
> this_cpu_ptr() and the local_irq_save() in unit_free().
>
> Thanks,
> David
>
> > Explicit migrate_disable() is only necessary from syscall.
> >
> > I believe rcu callbacks also cannot migrate, so the existing
> > code probably doesn't need them either.