netdev - Re: [RFC PATCH net] netns: fix GFP flags in rtnl_net

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Date:   Fri, 18 Oct 2019 13:55:46 -0700
From:   Pravin Shelar <pshelar@....org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Alexei Starovoitov <ast@...mgrid.com>,
        Jesse Gross <jesse@...ira.com>,
        Pravin B Shelar <pshelar@...ira.com>,
        Jiri Benc <jbenc@...hat.com>
Subject: Re: [RFC PATCH net] netns: fix GFP flags in rtnl_net_notifyid()

On Sun, Oct 13, 2019 at 3:22 PM Guillaume Nault <gnault@...hat.com> wrote:
>
> On Sun, Oct 13, 2019 at 12:09:43PM -0700, Pravin Shelar wrote:
> > On Thu, Oct 10, 2019 at 12:07 PM Guillaume Nault <gnault@...hat.com> wrote:
> > >
> > > In rtnl_net_notifyid(), we certainly can't pass a null GFP flag to
> > > rtnl_notify(). A GFP_KERNEL flag would be fine in most circumstances,
> > > but there are a few paths calling rtnl_net_notifyid() from atomic
> > > context or from RCU critical section. The later also precludes the use
> > > of gfp_any() as it wouldn't detect the RCU case. Also, the nlmsg_new()
> > > call is wrong too, as it uses GFP_KERNEL unconditionally.
> > >
> > > Therefore, we need to pass the GFP flags as parameter. The problem then
> > > propagates recursively to the callers until the proper flags can be
> > > determined. The problematic call chains are:
> > >
> > >  * ovs_vport_cmd_fill_info -> peernet2id_alloc -> rtnl_net_notifyid
> > >
> > >  * rtnl_fill_ifinfo -> rtnl_fill_link_netnsid -> peernet2id_alloc
> > >  -> rtnl_net_notifyid
> > >
> > > For openvswitch, ovs_vport_cmd_get() and ovs_vport_cmd_dump() prevent
> > > ovs_vport_cmd_fill_info() from using GFP_KERNEL. It'd be nice to move
> > > the call out of the RCU critical sections, but struct vport doesn't
> > > have a reference counter, so that'd probably require taking the ovs
> > > lock. Also, I don't get why ovs_vport_cmd_build_info() used GFP_ATOMIC
> > > in nlmsg_new(). I've changed it to GFP_KERNEL for consistency, as this
> > > functions seems to be allowed to sleep (as stated in the comment, it's
> > > called from a workqueue, under the protection of a mutex).
> > >
> > It is safe to change GFP flags to GFP_KERNEL in ovs_vport_cmd_build_info().
> > The patch looks good to me.
> >
> Thanks for your feedback.
>
> The point of my RFC is to know if it's possible to avoid all these
> gfp_t flags, by allowing ovs_vport_cmd_fill_info() to sleep (at least
> I'd like to figure out if it's worth spending time investigating this
> path).
>
> To do so, we'd requires moving the ovs_vport_cmd_fill_info() call of
> ovs_vport_cmd_{get,dump}() out of RCU critical section. Since we have
> no reference counter, I believe we'd have to protect these calls with
> ovs_lock() instead of RCU. Is that acceptable? If not, is there any
> other way?

I do not see point of added complexity and serialized OVS flow dumps
just to avoid GFP_ATOMIC allocations in some code path. What is issue
passing the parameter as you have done in this patch?