Re: [PATCH net] splice: do not checksum AF_UNIX sockets

[Frederik Deweerdt <deweerdt.lkml@gmail.com>]

On Tue, Dec 03, 2024 at 01:48:56PM -0800, Cong Wang wrote:
> On Tue, Dec 03, 2024 at 04:30:22PM +0000, David Howells wrote:
> > Frederik Deweerdt <deweerdt.lkml@gmail.com> wrote:
> > 
> > > -			if (skb->ip_summed == CHECKSUM_NONE)
> > > +			if (skb->ip_summed == CHECKSUM_NONE && skb->sk->sk_family != AF_UNIX)
> > >  				skb_splice_csum_page(skb, page, off, part);
> > 
> > Should AF_UNIX set some other CHECKSUM_* constant indicating that the checksum
> > is unnecessary?
> > 
> 
> It already means unnecessary on TX path:
> 
>  * - %CHECKSUM_NONE
>  *
>  *   The skb was already checksummed by the protocol, or a checksum is not
>  *   required.
> 

Looking back at the patch series that introduced the checksumming [1],
it looks like `ip_output.c::ip_append_page()` was the only path doing
checksumming, it had similar looking logic:
-
-		if (skb->ip_summed == CHECKSUM_NONE) {
-			__wsum csum;
-			csum = csum_page(page, offset, len);
-			skb->csum = csum_block_add(skb->csum, csum, skb->len);
-		}
-

That code in turn has been like this since the git import. I'm not sure
what that was for and how to test its intent.

Frederik

[1] https://patchwork.kernel.org/project/linux-mm/patch/20230522121125.2595254-15-dhowells@redhat.com/