netdev - Re: [RFC PATCH v3 08/12] net: support non paged skb frags

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Date: Tue, 7 Nov 2023 17:00:06 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Mina Almasry <almasrymina@...gle.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>, <linux-media@...r.kernel.org>,
	<dri-devel@...ts.freedesktop.org>, <linaro-mm-sig@...ts.linaro.org>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, Arnd Bergmann <arnd@...db.de>,
	David Ahern <dsahern@...nel.org>, Willem de Bruijn
	<willemdebruijn.kernel@...il.com>, Shuah Khan <shuah@...nel.org>, Sumit
 Semwal <sumit.semwal@...aro.org>, Christian König
	<christian.koenig@....com>, Shakeel Butt <shakeelb@...gle.com>, Jeroen de
 Borst <jeroendb@...gle.com>, Praveen Kaligineedi <pkaligineedi@...gle.com>
Subject: Re: [RFC PATCH v3 08/12] net: support non paged skb frags

On 2023/11/6 10:44, Mina Almasry wrote:
> Make skb_frag_page() fail in the case where the frag is not backed
> by a page, and fix its relevent callers to handle this case.
> 
> Correctly handle skb_frag refcounting in the page_pool_iovs case.
> 
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> 

...

>  /**
>   * skb_frag_page - retrieve the page referred to by a paged fragment
>   * @frag: the paged fragment
>   *
> - * Returns the &struct page associated with @frag.
> + * Returns the &struct page associated with @frag. Returns NULL if this frag
> + * has no associated page.
>   */
>  static inline struct page *skb_frag_page(const skb_frag_t *frag)
>  {
> -	return frag->bv_page;
> +	if (!page_is_page_pool_iov(frag->bv_page))
> +		return frag->bv_page;
> +
> +	return NULL;

It seems most of callers don't expect NULL returning for skb_frag_page(),
and this patch only changes a few relevant callers to handle the NULL case.

It may make more sense to add a new helper to do the above checking, and
add a warning in skb_frag_page() to catch any missing NULL checking for
skb_frag_page() caller, something like below?

 static inline struct page *skb_frag_page(const skb_frag_t *frag)
 {
-       return frag->bv_page;
+       struct page *page = frag->bv_page;
+
+       BUG_ON(page_is_page_pool_iov(page));
+
+       return page;
+}
+
+static inline struct page *skb_frag_readable_page(const skb_frag_t *frag)
+{
+       struct page *page = frag->bv_page;
+
+       if (!page_is_page_pool_iov(page))
+               return page;
+
+       return NULL;
 }