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

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

On Fri, Nov 29, 2024 at 05:43:04PM -0800, Cong Wang wrote:
> On Fri, Nov 29, 2024 at 03:20:14PM -0800, Frederik Deweerdt wrote:
> > When `skb_splice_from_iter` was introduced, it inadvertently added
> > checksumming for AF_UNIX sockets. This resulted in significant
> > slowdowns, as when using sendfile over unix sockets.
> > 
> > Using the test code [1] in my test setup (2G, single core x86_64 qemu),
> > the client receives a 1000M file in:
> > - without the patch: 1577ms (+/- 36.1ms)
> > - with the patch: 725ms (+/- 28.3ms)
> > 
> > This commit skips addresses the issue by skipping checksumming when
> > splice occurs a AF_UNIX socket.
> > 
> > [1] https://gist.github.com/deweerdt/a3ee2477d1d87524cf08618d3c179f06
> > 
> > Signed-off-by: Frederik Deweerdt <deweerdt.lkml@gmail.com>
> > Fixes: 2e910b95329c ("net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES")
> > ---
> >  net/core/skbuff.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 6841e61a6bd0..49e4f9ab625f 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -7233,7 +7233,7 @@ ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
> >  				goto out;
> >  			}
> >  
> > -			if (skb->ip_summed == CHECKSUM_NONE)
> > +			if (skb->ip_summed == CHECKSUM_NONE && skb->sk->sk_family != AF_UNIX)
> 
> Are you sure it is always safe to dereferene skb->sk here? I am not sure
> about the KCM socket case.
> 
> Instead of checking skb->sk->sk_family, why not just pass an additional
> boolean parameter to this function?
> 
I saw that every caller was dereferrencing an `sk`, but you are right
that that a boolean would be safer.

I'll resend an updated version.

Thanks,
Frederik