linux-kernel - Re: [PATCH net-next v4] net/core: fix wrong return value in __splice

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Message-ID: <689de6c51e152_18aa6c2948b@willemb.c.googlers.com.notmuch>
Date: Thu, 14 Aug 2025 09:38:13 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Pengtao He <hept.hept.hept@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Mina Almasry <almasrymina@...gle.com>, 
 Jason Xing <kerneljasonxing@...il.com>, 
 Michal Luczaj <mhal@...x.co>, 
 Eric Biggers <ebiggers@...gle.com>, 
 Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Pengtao He <hept.hept.hept@...il.com>
Subject: Re: [PATCH net-next v4] net/core: fix wrong return value in
 __splice_segment

Pengtao He wrote:
> If *len is equal to 0 at the beginning of __splice_segment
> it returns true directly. But when decreasing *len from
> a positive number to 0 in __splice_segment, it returns false.
> The caller needs to call __splice_segment again.
> 
> Recheck *len if it changes, return true in time.
> Reduce unnecessary calls to __splice_segment.

Fix is a strong term. The existing behavior is correct, it just takes
an extra pass through the loop in caller __skb_splice_bits. As also
indicated by this patch targeting net-next.

I would suggest something like "net: avoid one loop iteration in __skb_splice_bits"

> Signed-off-by: Pengtao He <hept.hept.hept@...il.com>
> ---
> v4:
> Correct the commit message.
> v3:
> Reduce once condition evaluation.
> v2:
> Correct the commit message and target tree.
> v1:
> https://lore.kernel.org/netdev/20250723063119.24059-1-hept.hept.hept@gmail.com/
> ---
>  net/core/skbuff.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index ee0274417948..23b776cd9879 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3112,7 +3112,9 @@ static bool __splice_segment(struct page *page, unsigned int poff,
>  		poff += flen;
>  		plen -= flen;
>  		*len -= flen;
> -	} while (*len && plen);
> +		if (!*len)
> +			return true;
> +	} while (plen);
>  
>  	return false;
>  }
> -- 
> 2.49.0
>