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Date: Mon, 15 Jan 2024 20:01:29 -0400
From: Jason Gunthorpe <jgg@nvidia.com>
To: Mina Almasry <almasrymina@google.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>, linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>,
	Shakeel Butt <shakeelb@google.com>,
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Subject: Re: [RFC PATCH net-next v5 2/2] net: add netmem to skb_frag_t
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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