Re: [PATCH bpf] Revert "xdp: add NULL pointer check in __xdp_return()"

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Fri, 10 Aug 2018 17:15:07 +0200, Björn Töpel wrote:
> Den fre 10 aug. 2018 kl 12:18 skrev Jesper Dangaard Brouer <brouer@redhat.com>:
> >
> > On Fri, 10 Aug 2018 11:28:02 +0200
> > Björn Töpel <bjorn.topel@gmail.com> wrote:
> >  
> > > From: Björn Töpel <bjorn.topel@intel.com>
> > >
> > > This reverts commit 36e0f12bbfd3016f495904b35e41c5711707509f.
> > >
> > > The reverted commit adds a WARN to check against NULL entries in the
> > > mem_id_ht rhashtable. Any kernel path implementing the XDP (generic or
> > > driver) fast path is required to make a paired
> > > xdp_rxq_info_reg/xdp_rxq_info_unreg call for proper function. In
> > > addition, a driver using a different allocation scheme than the
> > > default MEM_TYPE_PAGE_SHARED is required to additionally call
> > > xdp_rxq_info_reg_mem_model.
> > >
> > > For MEM_TYPE_ZERO_COPY, an xdp_rxq_info_reg_mem_model call ensures
> > > that the mem_id_ht rhashtable has a properly inserted allocator id. If
> > > not, this would be a driver bug. A NULL pointer kernel OOPS is
> > > preferred to the WARN.  
> >
> > Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
> >
> > As a comment says in the code: /* NB! Only valid from an xdp_buff! */
> > Which is (currently) guarded by the return/exit in convert_to_xdp_frame().
> >
> > This means that this code path can only be invoked while the driver is
> > still running under the RX NAPI process. Thus, there is no chance that
> > the allocator-id is gone (via calling xdp_rxq_info_unreg) for this code
> > path.
> >
> > But I really hope we at somepoint can convert a MEM_TYPE_ZERO_COPY into
> > a form of xdp_frame, that can travel further into the redirect-core.
> > In which case, we likely need to handle the NULL case (but also need
> > other code to handle what to do with the memory backing the frame)
> >
> > (I'm my vision here:)
> >
> > I really dislike that the current Zero-Copy mode steal ALL packets,
> > when ZC is enabled on a RX-queue.  This is not better than the existing
> > bypass solutions, which have ugly ways of re-injecting packet back into
> > the network stack.  With the integration with XDP, we have the
> > flexibility of selecting frames, that we don't want to be "bypassed"
> > into AF_XDP, and want the kernel process these. (The most common
> > use-case is letting the kernel handle the arptable).  IHMO this is what
> > will/would make AF_XDP superior to other bypass solutions.

Perhaps I'm misunderstanding, but I don't think that's necessarily
true.  AFAIU on XDP_PASS drivers should copy the frame into a skb and
pass it up the stack.  Granted that's fairly slow but *semantically*
AF_XDP doesn't necessarily steal all the packets :)

> Thanks for putting your visions/ideas here! I agree with both of your
> last sections, and this is what we're working towards. AF_XDP ZC has
> to play nicer with XDP.  The current (well, the soon-to-be-published
> [1] ;-)) ZC scheme is just a first step, and should be seen as a
> starting point so people can start playing using AF_XDP. Jakub also
> mentioned these issues a couple of threads ago, so there are
> definitely more people feeling the ZC allocator pains. Mid-term a
> sophisticated/proper and generic (for inter-driver reuse) ZC allocator
> is needed; Converting xdp_buffs to xdp_frames cheaply for multi-CPU
> completion, and hopefully dito for the XDP_PASS/kernel stack path. But
> let's start with something simple that works, and take it from there.
> 
> Björn
> 
> [1] WIP: https://github.com/bjoto/linux/tree/af-xdp-i40e-zc

Nice, looking forward to a refresh of i40e patches! :)

> > > Suggested-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > > Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
> > > ---
> > >  net/core/xdp.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > > index 6771f1855b96..9d1f22072d5d 100644
> > > --- a/net/core/xdp.c
> > > +++ b/net/core/xdp.c
> > > @@ -345,8 +345,7 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
> > >               rcu_read_lock();
> > >               /* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
> > >               xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
> > > -             if (!WARN_ON_ONCE(!xa))
> > > -                     xa->zc_alloc->free(xa->zc_alloc, handle);
> > > +             xa->zc_alloc->free(xa->zc_alloc, handle);
> > >               rcu_read_unlock();
> > >       default:
> > >               /* Not possible, checked in xdp_rxq_info_reg_mem_model() */  
> >
> >
> >
> > --
> > Best regards,
> >   Jesper Dangaard Brouer
> >   MSc.CS, Principal Kernel Engineer at Red Hat
> >   LinkedIn: http://www.linkedin.com/in/brouer