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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <64299af9e8861_2d2a20208e6@willemb.c.googlers.com.notmuch>
References: <64299af9e8861_2d2a20208e6@willemb.c.googlers.com.notmuch> <20230331160914.1608208-1-dhowells@redhat.com> <20230331160914.1608208-16-dhowells@redhat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     dhowells@redhat.com, Matthew Wilcox <willy@infradead.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jeff Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Chuck Lever III <chuck.lever@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 15/55] ip, udp: Support MSG_SPLICE_PAGES
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Date:   Mon, 03 Apr 2023 10:50:46 +0100
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Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> > +	} else if ((flags & MSG_SPLICE_PAGES) && length) {
> > +		if (inet->hdrincl)
> > +			return -EPERM;
> > +		if (rt->dst.dev->features & NETIF_F_SG)
> > +			/* We need an empty buffer to attach stuff to */
> > +			initial_length = transhdrlen;
> 
> I still don't entirely understand what initial_length means.
> 
> More importantly, transhdrlen can be zero. If not called for UDP
> but for RAW. Or if this is a subsequent call to a packet that is
> being held with MSG_MORE.
> 
> This works fine for existing use-cases, which go to alloc_new_skb.
> Not sure how this case would be different. But the comment alludes
> that it does.

The problem is that in the non-MSG_ZEROCOPY case, __ip_append_data() assumes
that it's going to copy the data it is given and will allocate sufficient
space in the skb in advance to hold it - but I don't want to do that because I
want to splice in the pages holding the data instead.  However, I do need to
allocate space to hold the transport header.

Maybe I should change 'initial_length' to 'initial_alloc'?  It represents the
amount I think we should allocate.  Or maybe I should have a separate
allocation clause for MSG_SPLICE_PAGES?

I also wonder if __ip_append_data() really needs two places that call
getfrag().

David