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

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Date: Wed, 8 Nov 2023 19:25:16 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Mina Almasry <almasrymina@...gle.com>
CC: <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>, "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 2023/11/8 5:19, Mina Almasry wrote:
>>
>>
> 
> My personal immediate reaction is that this may just introduce code
> churn without significant benefit. If an unsuspecting caller call
> skb_frag_page() on devmem frag and doesn't correctly handle NULL
> return, it will crash or error out anyway, and likely in some obvious
> way, so maybe the BUG_ON() isn't so useful that it's worth changing

If it will always crash or error out, then I agree that BUG_ON() is
unnecessary.

> all the call sites. But if there is consensus on adding a change like
> you propose, I have no problem adding it.

One obvious benefit I forget to mention is that, it provides a better
semantic that if a caller need to do the return checking:
1. For the old helper, the semantic is not to do the checking if the caller
   has ensure that it has passed a readable frag to skb_frag_page(), which
   avoid adding some overhead for non-devmen supported drivers.
2. For the new helper, the semantic is to do the checking and we may provide
   a compiler '__must_check' function-attribute to ensure the caller to do
   the checking.


>