netdev - Re: [PATCH net] splice: do not checksum AF

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Message-ID: <Z0ptqDcLjrjqruQA@pop-os.localdomain>
Date: Fri, 29 Nov 2024 17:43:04 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Frederik Deweerdt <deweerdt.lkml@...il.com>
Cc: netdev@...r.kernel.org, dhowells@...hat.com
Subject: Re: [PATCH net] splice: do not checksum AF_UNIX sockets

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@...il.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?

Thanks.