netdev - Re: [RFC PATCH net-next v5 2/2] net: add netmem to skb_frag

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Message-ID: <20240116000129.GX734935@nvidia.com>
Date: Mon, 15 Jan 2024 20:01:29 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Yunsheng Lin <linyunsheng@...wei.com>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Christian König <christian.koenig@....com>,
	Shakeel Butt <shakeelb@...gle.com>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [RFC PATCH net-next v5 2/2] net: add netmem to skb_frag_t

On Mon, Jan 15, 2024 at 03:23:33PM -0800, Mina Almasry wrote:
> > > You did not answer my question that I asked here, and ignoring this
> > > question is preventing us from making any forward progress on this
> > > discussion. What do you expect or want skb_frag_page() to do when
> > > there is no page in the frag?
> >
> > I would expect it to do nothing.
> 
> I don't understand. skb_frag_page() with an empty implementation just
> results in a compiler error as the function needs to return a page
> pointer. Do you actually expect skb_frag_page() to unconditionally
> cast frag->netmem to a page pointer? That was explained as
> unacceptable over and over again by Jason and Christian as it risks
> casting devmem to page; completely unacceptable and will get nacked.
> Do you have a suggestion of what skb_frag_page() should do that will
> not get nacked by mm?

WARN_ON and return NULL seems reasonable?

Jason