linux-kernel - Re: [RFC PATCH 3/3] net: ethernet: ti: cpsw: add XDP support

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Date:   Fri, 19 Apr 2019 13:42:30 +0300
From:   Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc:     grygorii.strashko@...com, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        hawk@...nel.org, xdp-newbies@...r.kernel.org, ast@...nel.org,
        aniel@...earbox.net, jakub.kicinski@...ronome.com,
        john.fastabend@...il.com
Subject: Re: [RFC PATCH 3/3] net: ethernet: ti: cpsw: add XDP support

On Fri, Apr 19, 2019 at 11:31:56AM +0300, Ilias Apalodimas wrote:
>Hi Ivan,
>
>> +static struct page *cpsw_alloc_page(struct cpsw_common *cpsw)
>> +{
>> +	struct page_pool *pool = cpsw->rx_page_pool;
>> +	struct page *page;
>> +	int i = 0;
>> +
>> +	do {
>> +		page = page_pool_dev_alloc_pages(pool);
>> +		if (!page)
>> +			return NULL;
>> +
>> +		/* skip pages used by skb netstack */
>I think the comment here is wrong and might confuse people.
>The page ref cnt is 1, which means the packet was *processed* and netstack is
>done with it, hence you can re-use it.
>If it's !=1 then you correctly unmap the buffer and decrease the ref cnt, so it
>will eventually be freed and not returned to the pool, right?
It's compensation substitution for page_pool support in skb netsack.
And should be considered in combine with:

skb = build_skb(xmeta, cpsw_rxbuf_total_len(pkt_size));
...
page_pool_recycle_direct(pool, page);
page_ref_inc(page);
netif_receive_skb(skb);

Here order is important.

I will correct comments in final version (w/o overloading) ofc, leaving
thinking environment for people. I think it's fair enough about this.

>> +		if (page_ref_count(page) == 1)
>> +			break;
>> +
>> +		/* it's a pitty, but free page */
>> +		page_pool_recycle_direct(pool, page);
>> +	} while (++i < descs_pool_size);
>> +
>> +	return page;
>> +}
>> +
>
> /Ilias

-- 
Regards,
Ivan Khoronzhuk