netdev - Re: NET_DMA: where do we ever call dma_skb_copy_datagram_iovec() with NULL pinned

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Message-ID: <c0a09e5c0707241024p3533df25h5e8dc904d0e0778f@mail.gmail.com>
Date:	Tue, 24 Jul 2007 10:24:54 -0700
From:	"Andrew Grover" <andy.grover@...il.com>
To:	"Al Viro" <viro@....linux.org.uk>, shannon.nelson@...el.com
Cc:	netdev@...r.kernel.org, christopher.leech@...el.com
Subject: Re: NET_DMA: where do we ever call dma_skb_copy_datagram_iovec() with NULL pinned_list?

On 7/20/07, Al Viro <viro@....linux.org.uk> wrote:
>         AFAICS, all callers of dma_skb_copy_datagram_iovec()
> are either
>         * recursive for fragments, pass pinned_list unchanged or
>         * called from tcp, with pinned_list coming from
> tp->ucopy.pinned_list and only when tp->ucopy.dma_chan is non-NULL.
>
> Now, all non-NULL assignments to ->dma_chan have the same form:
>         if (!tp->ucopy.dma_chan && tp->ucopy.pinned_list)
>                 tp->ucopy.dma_chan = get_softnet_dma();
> IOW, if ->ucopy.pinned_list stays NULL, ->ucopy.dma_chan will do the same.
>
> Moreover, any place that resets ->ucopy.pinned_list will also reset
> ->ucopy.dma_chan.
>
> IOW, we can't ever get non-NULL tp->ucopy.dma_chan while tp->ucopy.pinned_list
> is NULL.  So how can we ever get to the dma_memcpy_to_kernel_iovec()?

Shannon what do you think? Is this a bug, or does it no longer support
non-userspace iovecs?

Thanks -- Andy
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