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

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Date:   Tue, 2 Jul 2019 14:37:39 +0300
From:   Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     grygorii.strashko@...com, davem@...emloft.net, ast@...nel.org,
        linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
        ilias.apalodimas@...aro.org, netdev@...r.kernel.org,
        daniel@...earbox.net, jakub.kicinski@...ronome.com,
        john.fastabend@...il.com
Subject: Re: [PATCH v5 net-next 6/6] net: ethernet: ti: cpsw: add XDP support

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 <ivan.khoronzhuk@...aro.org> 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.

>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/20190625175948.24771-2-ivan.khoronzhuk@linaro.org/
>
>
>> +void cpsw_ndev_destroy_xdp_rxqs(struct cpsw_priv *priv)
>> +{
>> +	struct cpsw_common *cpsw = priv->cpsw;
>> +	struct xdp_rxq_info *rxq;
>> +	int i;
>> +
>> +	for (i = 0; i < cpsw->rx_ch_num; i++) {
>> +		rxq = &priv->xdp_rxq[i];
>> +		if (xdp_rxq_info_is_reg(rxq))
>> +			xdp_rxq_info_unreg(rxq);
>> +	}
>> +}
>
>Are you sure you need to test xdp_rxq_info_is_reg() here?
Yes it's required in my case as it's used in error path where
an rx queue can be even not registered and no need in this warn.

>
>You should just call xdp_rxq_info_unreg(rxq), if you know that this rxq
>should be registered.  If your assumption failed, you will get a
>WARNing, and discover your driver level bug.  This is one of the ways
>the API is designed to "detect" misuse of the API.  (I found this
>rather useful, when I converted the approx 12 drivers using this
>xdp_rxq_info API).
>
>-- 
>Best regards,
>  Jesper Dangaard Brouer
>  MSc.CS, Principal Kernel Engineer at Red Hat
>  LinkedIn: http://www.linkedin.com/in/brouer

-- 
Regards,
Ivan Khoronzhuk