Subject: Re: XDP_TX bug report on mlx4

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 16 Sep 2016 21:24:43 +0200

On Fri, 16 Sep 2016 12:17:27 -0700
Brenden Blanco <[email-masked]> wrote:

> On Fri, Sep 16, 2016 at 09:03:40PM +0200, Jesper Dangaard Brouer wrote:
> > Hi Brenden,
> > 
> > I've discovered a bug with XDP_TX recycling of pages in the mlx4 driver.
> > 
> > If I increase the number of RX and TX queues/channels via ethtool cmd:
> >  ethtool -L mlx4p1 rx 10 tx 10
> > 
> > Then when running the xdp2 program, which does XDP_TX, the kernel will
> > crash with page errors, because the page refcnt goes to zero or even
> > minus.  I've noticed pages delivered to mlx4_en_rx_recycle() can have
> > a page refcnt of zero, which is wrong, they should always have 1 (for
> > XDP).
> > 
> > Debugging it further, I find that this can happen when mlx4_en_rx_recycle()
> > is called from mlx4_en_recycle_tx_desc().  This is the TX cleanup function,
> > associated with TX ring queues used for XDP_TX only. No others than the
> > XDP_TX action should be able to place packets into these TX rings
> > which call mlx4_en_recycle_tx_desc().  
> 
> Sounds pretty straightforward, let me look into it.

Here is some debug info I instrumented my kernel with, and I've
attached my minicom output with a warning and a panic.

Enable some driver debug printks via::
 ethtool -s mlx4p1 msglvl drv on

Debug normal situation::

 $ grep recycle_ring minicom_capturefile.log08
 [  520.746610] mlx4_en: mlx4p1: Set tx_ring[56]->recycle_ring = rx_ring[0]
 [  520.747042] mlx4_en: mlx4p1: Set tx_ring[57]->recycle_ring = rx_ring[1]
 [  520.747470] mlx4_en: mlx4p1: Set tx_ring[58]->recycle_ring = rx_ring[2]
 [  520.747918] mlx4_en: mlx4p1: Set tx_ring[59]->recycle_ring = rx_ring[3]
 [  520.748330] mlx4_en: mlx4p1: Set tx_ring[60]->recycle_ring = rx_ring[4]
 [  520.748749] mlx4_en: mlx4p1: Set tx_ring[61]->recycle_ring = rx_ring[5]
 [  520.749181] mlx4_en: mlx4p1: Set tx_ring[62]->recycle_ring = rx_ring[6]
 [  520.749620] mlx4_en: mlx4p1: Set tx_ring[63]->recycle_ring = rx_ring[7]

Change $ ethtool -L mlx4p1 rx 9 tx 9 ::

 [  911.594692] mlx4_en: mlx4p1: Set tx_ring[56]->recycle_ring = rx_ring[0]
 [  911.608345] mlx4_en: mlx4p1: Set tx_ring[57]->recycle_ring = rx_ring[1]
 [  911.622008] mlx4_en: mlx4p1: Set tx_ring[58]->recycle_ring = rx_ring[2]
 [  911.636364] mlx4_en: mlx4p1: Set tx_ring[59]->recycle_ring = rx_ring[3]
 [  911.650015] mlx4_en: mlx4p1: Set tx_ring[60]->recycle_ring = rx_ring[4]
 [  911.663690] mlx4_en: mlx4p1: Set tx_ring[61]->recycle_ring = rx_ring[5]
 [  911.677356] mlx4_en: mlx4p1: Set tx_ring[62]->recycle_ring = rx_ring[6]
 [  911.690924] mlx4_en: mlx4p1: Set tx_ring[63]->recycle_ring = rx_ring[7]
 [  911.704544] mlx4_en: mlx4p1: Set tx_ring[64]->recycle_ring = rx_ring[8]
 [  911.718171] mlx4_en: mlx4p1: Set tx_ring[65]->recycle_ring = rx_ring[9]
 [  911.731772] mlx4_en: mlx4p1: Set tx_ring[66]->recycle_ring = rx_ring[10]
 [  911.745438] mlx4_en: mlx4p1: Set tx_ring[67]->recycle_ring = rx_ring[11]
 [  911.759063] mlx4_en: mlx4p1: Set tx_ring[68]->recycle_ring = rx_ring[12]
 [  911.772741] mlx4_en: mlx4p1: Set tx_ring[69]->recycle_ring = rx_ring[13]
 [  911.786415] mlx4_en: mlx4p1: Set tx_ring[70]->recycle_ring = rx_ring[14]
 [  911.800070] mlx4_en: mlx4p1: Set tx_ring[71]->recycle_ring = rx_ring[15]

Change $ ethtool -L mlx4p1 rx 10 tx 10::

 netif_set_real_num_tx_queues() setting dev->real_num_tx_queues(now:80) = 64
 mlx4_en: mlx4p1:   frag:0 - size:1522 prefix:0 stride:4096
 mlx4_en_init_recycle_ring() Set tx_ring[64]->recycle_ring = rx_ring[0]
 mlx4_en_init_recycle_ring() Set tx_ring[65]->recycle_ring = rx_ring[1]
 mlx4_en_init_recycle_ring() Set tx_ring[66]->recycle_ring = rx_ring[2]
 mlx4_en_init_recycle_ring() Set tx_ring[67]->recycle_ring = rx_ring[3]
 mlx4_en_init_recycle_ring() Set tx_ring[68]->recycle_ring = rx_ring[4]
 mlx4_en_init_recycle_ring() Set tx_ring[69]->recycle_ring = rx_ring[5]
 mlx4_en_init_recycle_ring() Set tx_ring[70]->recycle_ring = rx_ring[6]
 mlx4_en_init_recycle_ring() Set tx_ring[71]->recycle_ring = rx_ring[7]
 mlx4_en_init_recycle_ring() Set tx_ring[72]->recycle_ring = rx_ring[8]
 mlx4_en_init_recycle_ring() Set tx_ring[73]->recycle_ring = rx_ring[9]
 mlx4_en_init_recycle_ring() Set tx_ring[74]->recycle_ring = rx_ring[10]
 mlx4_en_init_recycle_ring() Set tx_ring[75]->recycle_ring = rx_ring[11]
 mlx4_en_init_recycle_ring() Set tx_ring[76]->recycle_ring = rx_ring[12]
 mlx4_en_init_recycle_ring() Set tx_ring[77]->recycle_ring = rx_ring[13]
 mlx4_en_init_recycle_ring() Set tx_ring[78]->recycle_ring = rx_ring[14]
 mlx4_en_init_recycle_ring() Set tx_ring[79]->recycle_ring = rx_ring[15]

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer