Re: [PATCH 4/6] tcp_splice: Simplify tracking of read/written bytes

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 20 May 2026 23:08:49 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> For each each direction of each spliced connection, we keep track of how
> many bytes we've read from one socket and written to the other.  However,
> we never actually care about the absolute values of these, only the
> difference between them, which represents how much data is currently "in
> flight" in the splicing pipe.
> 
> Simplify the handling by having a single variable tracking the number of
> bytes in the pipe.

For me it actually looks slightly more complicated to think about it
this way, I added explicit 'read' and 'written' after being bitten by
some issue I introduced with a previous 'pending' concept, but I have
to admit it slightly simplifies the overflow topic.

> As a bonus, the new scheme makes it clearer that we don't need to worry
> about overflows: pending can never become larger than the maximum pipe
> bufffer size, well within 32-bits.
> 
> I _think_ the old scheme was safe in the case of overflow - again under
> the assumption that read/written can never be further apart than the pipe
> buffer size.  However, it's much harder to reason about this case.  It's
> certainly plausible that an overflow could occur - sending 4GiB through
> a local socket is entirely achievable.

For me it looked pretty simple: you can overflow 32 bits (at 100 Gbps,
but without hitting the "optimised" case, it would take about five
minutes), but all the operations between the two counters are between
two uint32_t, so they would happen in uint32_t, hence modulo 32 bits,
similar to TCP sequences.

Anyway, overall, I think it's an improvement over the original. One nit
here:

> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tcp_conn.h   |  6 ++----
>  tcp_splice.c | 18 +++++++++---------
>  2 files changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/tcp_conn.h b/tcp_conn.h
> index 9f5bee03..c8381aa7 100644
> --- a/tcp_conn.h
> +++ b/tcp_conn.h
> @@ -206,8 +206,7 @@ struct tcp_tap_transfer_ext {
>   * @f:			Generic flow information
>   * @s:			File descriptor for sockets
>   * @pipe:		File descriptors for pipes
> - * @read:		Bytes read (not fully written to other side in one shot)
> - * @written:		Bytes written (not fully written from one other side read)
> + * @pending:		Bytes currently in each pipe
>   * @events:		Events observed/actions performed on connection
>   * @flags:		Connection flags (attributes, not events)
>   */
> @@ -218,8 +217,7 @@ struct tcp_splice_conn {
>  	int s[SIDES];
>  	int pipe[SIDES][2];
>  
> -	uint32_t read[SIDES];
> -	uint32_t written[SIDES];
> +	uint32_t pending[SIDES];
>  
>  	uint8_t events;
>  #define SPLICE_CLOSED			0
> diff --git a/tcp_splice.c b/tcp_splice.c
> index 18e8b303..8fbd490f 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -292,7 +292,7 @@ bool tcp_splice_flow_defer(struct tcp_splice_conn *conn)
>  			conn->s[sidei] = -1;
>  		}
>  
> -		conn->read[sidei] = conn->written[sidei] = 0;
> +		conn->pending[sidei] = 0;
>  	}
>  
>  	conn->events = SPLICE_CLOSED;
> @@ -490,7 +490,7 @@ static int tcp_splice_forward(struct ctx *c, struct
>  	int eof = 0;
>  
>  	while (1) {
> -		ssize_t readlen, written, pending;
> +		ssize_t readlen, written;
>  		int more = 0;
>  
>  retry:
> @@ -537,7 +537,7 @@ retry:
>  		flow_trace(conn, "%zi from write-side call (passed %zi)",
>  			   written, c->tcp.pipe_size);
>  
> -		/* Most common case: skip updating counters. */
> +		/* Most common case: skip updating pending. */

"pending" isn't a noun (even though the variable name is, but it's
not quite obvious that you're referring to it). I think that:

		/* Most common case: skip updating count of pending bytes */

would be slightly clearer (and also omit the '.' because it's not a
complete sentence, as we usually do on single-line comments, similarly
to most occurrences in the kernel).

>  		if (readlen > 0 && readlen == written) {
>  			if (readlen >= (long)c->tcp.pipe_size * 10 / 100)
>  				continue;
> @@ -561,11 +561,11 @@ retry:
>  			continue;
>  		}
>  
> -		conn->read[fromsidei]    += readlen > 0 ? readlen : 0;
> -		conn->written[fromsidei] += written > 0 ? written : 0;
> +		conn->pending[fromsidei] += readlen > 0 ? readlen : 0;
> +		conn->pending[fromsidei] -= written > 0 ? written : 0;
>  
>  		if (written < 0) {
> -			if (conn->read[fromsidei] == conn->written[fromsidei])
> +			if (!conn->pending[fromsidei])
>  				break;
>  
>  			conn_event(conn, OUT_WAIT(!fromsidei));
> @@ -575,15 +575,15 @@ retry:
>  		if (never_read && written == (long)(c->tcp.pipe_size))
>  			goto retry;
>  
> -		pending = conn->read[fromsidei] - conn->written[fromsidei];
> -		if (!never_read && written > 0 && written < pending)
> +		if (!never_read && written > 0 &&
> +		    written < conn->pending[fromsidei])
>  			goto retry;
>  
>  		if (eof)
>  			break;
>  	}
>  
> -	if (conn->read[fromsidei] == conn->written[fromsidei] && eof) {
> +	if (!conn->pending[fromsidei] && eof) {
>  		unsigned sidei;
>  
>  		flow_foreach_sidei(sidei) {

-- 
Stefano