Subject: Re: [PATCH v5 net-next 6/6] net: ethernet: ti: cpsw: add XDP support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 2 Jul 2019 15:39:02 +0200

On Tue, 2 Jul 2019 14:37:39 +0300
Ivan Khoronzhuk <[email-masked]> wrote:

> On Mon, Jul 01, 2019 at 06:19:01PM +0200, Jesper Dangaard Brouer wrote:
> >On Sun, 30 Jun 2019 20:23:48 +0300
> >Ivan Khoronzhuk <[email-masked]> wrote:
> >  
> >> +static int cpsw_ndev_create_xdp_rxq(struct cpsw_priv *priv, int ch)
> >> +{
> >> +	struct cpsw_common *cpsw = priv->cpsw;
> >> +	int ret, new_pool = false;
> >> +	struct xdp_rxq_info *rxq;
> >> +
> >> +	rxq = &priv->xdp_rxq[ch];
> >> +
> >> +	ret = xdp_rxq_info_reg(rxq, priv->ndev, ch);
> >> +	if (ret)
> >> +		return ret;
> >> +
> >> +	if (!cpsw->page_pool[ch]) {
> >> +		ret =  cpsw_create_rx_pool(cpsw, ch);
> >> +		if (ret)
> >> +			goto err_rxq;
> >> +
> >> +		new_pool = true;
> >> +	}
> >> +
> >> +	ret = xdp_rxq_info_reg_mem_model(rxq, MEM_TYPE_PAGE_POOL,
> >> +					 cpsw->page_pool[ch]);
> >> +	if (!ret)
> >> +		return 0;
> >> +
> >> +	if (new_pool) {
> >> +		page_pool_free(cpsw->page_pool[ch]);
> >> +		cpsw->page_pool[ch] = NULL;
> >> +	}
> >> +
> >> +err_rxq:
> >> +	xdp_rxq_info_unreg(rxq);
> >> +	return ret;
> >> +}  
> >
> >Looking at this, and Ilias'es XDP-netsec error handling path, it might
> >be a mistake that I removed page_pool_destroy() and instead put the
> >responsibility on xdp_rxq_info_unreg().  
>
> As for me this is started not from page_pool_free, but rather from calling
> unreg_mem_model from rxq_info_unreg. Then, if page_pool_free is hidden
> it looks more a while normal to move all chain to be self destroyed.
> 
> >
> >As here, we have to detect if page_pool_create() was a success, and then
> >if xdp_rxq_info_reg_mem_model() was a failure, explicitly call
> >page_pool_free() because the xdp_rxq_info_unreg() call cannot "free"
> >the page_pool object given it was not registered.  
>
> Yes, it looked a little bit ugly from the beginning, but, frankly,
> I have got used to this already.
> 
> >
> >Ivan's patch in[1], might be a better approach, which forced all
> >drivers to explicitly call page_pool_free(), even-though it just
> >dec-refcnt and the real call to page_pool_free() happened via
> >xdp_rxq_info_unreg().
> >
> >To better handle error path, I would re-introduce page_pool_destroy(),
>
> So, you might to do it later as I understand, and not for my special
> case but becouse it makes error path to look a little bit more pretty.
> I'm perfectly fine with this, and better you add this, for now my
> implementation requires only "xdp: allow same allocator usage" patch,
> but if you insist I can resend also patch in question afterwards my
> series is applied (with modification to cpsw & netsec & mlx5 & page_pool).
> 
> What's your choice? I can add to your series patch needed for cpsw to
> avoid some misuse.

I will try to create a cleaned-up version of your patch[1] and
re-introduce page_pool_destroy() for drivers to use, then we can build
your driver on top of that.

> >as a driver API, that would gracefully handle NULL-pointer case, and
> >then call page_pool_free() with the atomic_dec_and_test().  (It should
> >hopefully simplify the error handling code a bit)
> >
> >[1] https://lore.kernel.org/netdev/[email-masked]/
[...]

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