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 09:09:20 -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 8:42 AM David Vernet <void@...ifault.com> wrote:
>
> On Fri, Jun 23, 2023 at 08:13:33PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@...nel.org>
> >
> > Convert bpf_cpumask to bpf_mem_cache_free_rcu.
> >
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>
> Acked-by: David Vernet <void@...ifault.com>
>
> LGTM, thanks for cleaning this up. I left one drive-by comment /
> observation below, but it's not a blocker for this patch / series.
>
> > ---
> >  kernel/bpf/cpumask.c | 20 ++++++--------------
> >  1 file changed, 6 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> > index 938a60ff4295..6983af8e093c 100644
> > --- a/kernel/bpf/cpumask.c
> > +++ b/kernel/bpf/cpumask.c
> > @@ -9,7 +9,6 @@
> >  /**
> >   * struct bpf_cpumask - refcounted BPF cpumask wrapper structure
> >   * @cpumask: The actual cpumask embedded in the struct.
> > - * @rcu:     The RCU head used to free the cpumask with RCU safety.
> >   * @usage:   Object reference counter. When the refcount goes to 0, the
> >   *           memory is released back to the BPF allocator, which provides
> >   *           RCU safety.
> > @@ -25,7 +24,6 @@
> >   */
> >  struct bpf_cpumask {
> >       cpumask_t cpumask;
> > -     struct rcu_head rcu;
> >       refcount_t usage;
> >  };
> >
> > @@ -82,16 +80,6 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_acquire(struct bpf_cpumask *cpumask)
> >       return cpumask;
> >  }
> >
> > -static void cpumask_free_cb(struct rcu_head *head)
> > -{
> > -     struct bpf_cpumask *cpumask;
> > -
> > -     cpumask = container_of(head, struct bpf_cpumask, rcu);
> > -     migrate_disable();
> > -     bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
> > -     migrate_enable();
> > -}
> > -
> >  /**
> >   * bpf_cpumask_release() - Release a previously acquired BPF cpumask.
> >   * @cpumask: The cpumask being released.
> > @@ -102,8 +90,12 @@ static void cpumask_free_cb(struct rcu_head *head)
> >   */
> >  __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
> >  {
> > -     if (refcount_dec_and_test(&cpumask->usage))
> > -             call_rcu(&cpumask->rcu, cpumask_free_cb);
> > +     if (!refcount_dec_and_test(&cpumask->usage))
> > +             return;
> > +
> > +     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.
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.