Re: [PATCH 5/6] tcp_splice: Simplify EPOLLRDHUP / eof / FIN handling

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, May 21, 2026 at 09:15:13AM +0200, Stefano Brivio wrote:
> On Thu, 21 May 2026 16:56:43 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, May 21, 2026 at 07:40:31AM +0200, Stefano Brivio wrote:
> > > On Thu, 21 May 2026 12:03:33 +1000
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Wed, May 20, 2026 at 10:30:04PM +0200, Stefano Brivio wrote:  
> > > > > On Wed, 20 May 2026 23:08:50 +1000
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >     
> > > > > > There are two ways we can tell one of our sockets has received a FIN.  We
> > > > > > can either see an EPOLLRDHUP epoll event, or we can get a zero-length read
> > > > > > (EOF) on the socket.  We currently use both, in a mildly confusing way:
> > > > > > we only set the FIN_RCVD() flag based on the EPOLLRDHUP event, but then
> > > > > > some other close out logic is based on seeing an EOF.
> > > > > > 
> > > > > > Simplify this by setting the flag based on only the EOF.  To make sure we
> > > > > > don't miss an event if we get an EPOLLRDHUP with no data, we trigger the
> > > > > > forwarding path for EPOLLRDHUP as well as EPOLLIN.
> > > > > > 
> > > > > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > > > > ---
> > > > > >  tcp_splice.c | 14 +++++---------
> > > > > >  1 file changed, 5 insertions(+), 9 deletions(-)
> > > > > > 
> > > > > > diff --git a/tcp_splice.c b/tcp_splice.c
> > > > > > index 8fbd490f..b45f0060 100644
> > > > > > --- a/tcp_splice.c
> > > > > > +++ b/tcp_splice.c
> > > > > > @@ -487,7 +487,6 @@ static int tcp_splice_forward(struct ctx *c, struct
> > > > > >  	uint8_t lowat_set_flag = RCVLOWAT_SET(fromsidei);
> > > > > >  	uint8_t lowat_act_flag = RCVLOWAT_ACT(fromsidei);
> > > > > >  	int never_read = 1;
> > > > > > -	int eof = 0;
> > > > > >  
> > > > > >  	while (1) {
> > > > > >  		ssize_t readlen, written;
> > > > > > @@ -510,7 +509,7 @@ retry:
> > > > > >  		flow_trace(conn, "%zi from read-side call", readlen);
> > > > > >  
> > > > > >  		if (!readlen) {
> > > > > > -			eof = 1;
> > > > > > +			conn_event(conn, FIN_RCVD(fromsidei));    
> > > > > 
> > > > > I'm not sure if I really found a concrete issue with this, but it looks
> > > > > a bit scary, because it changes the semantics of FIN_RCVD, which used to
> > > > > mean that we infer we received a FIN, regardless of whether we're done
> > > > > processing all data from that half of the connection.
> > > > > 
> > > > > Now FIN_RCVD is only set if we actually processed all the data and we
> > > > > hit the end of file.    
> > > > 
> > > > True.  But the only place that tested FIN_RCVD was at the end of
> > > > tcp_splice_forward(), conditional on 'eof' anyway.  In a sense, this
> > > > was the cause of bug202 - we had FIN_RCVD set, but we didn't process
> > > > it and shutdown() on the other side, because we didn't have eof.  
> > > 
> > > That sounds like a good motivation to clean this up, just two concerns
> > > below:
> > >   
> > > > > The (potential) issue I see here is that we get EPOLLRDHUP, splice()
> > > > > returns -1 with EAGAIN in errno because we had no room in the pipe,
> > > > > and it would have returned 0 instead.
> > > > > 
> > > > > Will we ever get our zero-sized "read" later? If not, we might have
> > > > > missed EPOLLRDHUP *and* the end of file. I'm not entirely sure we have
> > > > > guarantees in that sense from splice().    
> > > > 
> > > > It's not really about guarantees from splice.  I'm pretty sure this is
> > > > ok, reasoning as follows.
> > > > 
> > > > Consider all the exit points from the loop body:
> > > >  - Each return is a return -1, so we kill the connection anyway.  They
> > > >    don't matter
> > > >  - Each continue, goto retry and the end of the body will do the read
> > > >    side splice() again, so get another chance to see the EOF
> > > >  - That leaves just the breaks
> > > > 
> > > > Consider each break (there are three, since patch 2 of this series)
> > > > 		if (written < 0) {
> > > > 			if (!conn->pending[fromsidei])
> > > > 				break;
> > > > 
> > > > (1) The pipe is empty and the write-splice returned EAGAIN, so it
> > > > didn't remove data from the pipe.  
> > > 
> > > You're assuming that !conn->pending[fromsidei] means that the pipe is
> > > empty. From what we see of it, it is.  
> > 
> > It does mean the pipe is empty.  Everything we put in, we've taken
> > out.  There cannot be anything in there.
> > 
> > > What the kernel can do with it, though, is different. It might return
> > > EAGAIN even if we think we should have space, because it's resizing it
> > > under memory pressure or anything like that. Or it delays freeing up
> > > space or accounting for whatever reason.  
> > 
> > Theoretically, I suppose.  But !pending doesn't just mean the pipe is
> > not full it means it's completely completely empty.  Not being able to
> > put any bytes at all into an empty pipe would be *very* surprising.
> > So much so that if it happened in practice, I suspect we wouldn't be
> > safe not having epoll events on the pipe ends, so that we can be
> > notified when it deigns to accept some data.
> 
> We can get 512-byte pipes, I actually saw that happening in practice
> with either:

Sure.. so?  We can still put some bytes into it if it's empty.

> - people setting low values for ulimits
> 
> - the user (or just pasta itself) having a lot of pipes open
> 
> and if I recall correctly that's where I saw the case of a supposedly
> empty pipe giving us EAGAIN. That was years ago though and I didn't
> specifically fix that.

I mean.. that sounds like a kernel bug.  If we do have to handle that
case we'll need epoll events on the pipe ends, since none of the
socket events we monitor will trigger when the pipe becomes writable.

> We currently probe the size based on the value we can have for 32 pipes
> (TCP_SPLICE_PIPE_POOL_SIZE). By making that 4096 or so you should get
> rather small pipes.
> 
> Things might already be broken with them, I haven't checked the
> behaviour in a long while. I think 512 bytes was the lower bound I hit.
> 
> > > So it would be nice to make this part robust to that. I thought setting
> > > FIN_RCVD on EPOLLRDHUP was a good way to achieve that.
> > >   
> > > >  Therefore, the pipe must have been
> > > > empty before the write-splice.  Which means the read-splice can't have
> > > > blocked on a full pipe.
> > > > 			conn_event(conn, OUT_WAIT(!fromsidei));
> > > > 			break;
> > > > 		}
> > > > 
> > > > (2) The pipe is non-empty and the write-splice returned EAGAIN, so it
> > > > must have blocked on the output socket.  We've set OUT_WAIT(), so
> > > > we'll get an EPOLLOUT at some point which will cause us to read-splice
> > > > again, meaning we get another chance to see the EOF.  
> > > 
> > > ...later. But what if we don't get a zero-sized read *at all*? I'm not
> > > sure if splice() guarantees we do get one if we reach end-of-file.  
> > 
> > > That's something valid and very well established for read() and recv(),
> > > but splice() is a bit weird. The documentation says:
> > > 
> > >   A return value of 0 means end of input.
> > > 
> > > but I wouldn't assume we'll *always* get at least one in case of EOF.  
> > 
> > What else could we plausibly get?
> 
> -1 with EBADF, probably with EPOLLERR, because something timed out?

EBADF makes no sense, the fds are still valid, even if they're at EOF.

> But I guess you're right, as long as we're not in the EPOLLERR category
> of things, we should consistently get 0, even if we read multiple
> times.
> 
> I had in mind some kernel behaviour where we get 0 once, and then -1
> (EAGAIN?) because... go figure. But no, it can't happen.

I think the logic should be ok as long as we see a 0 once, even if we
get EAGAINs after that.

Another way to look at this - if we had to monitor EPOLLRDHUP to get
this right, splice() would be unusable from blocking / synchronous
code, which I don't think is the case.

> 
> > > > 		[...]
> > > > 		if (conn->events & FIN_RCVD(fromsidei))
> > > > 			break;
> > > > (3) By the new semantics of FIN_RCVD, we *have* seen the EOF.
> > > >   
> > > > > The existing implementation distinguishes between end-of-file we hit in
> > > > > a given iteration, and EPOLLRDHUP we might have seen at any time.
> > > > > That was actually intended.    
> > > > 
> > > > It might be intended, but I can't see that we did anything with that
> > > > information.  
> > > 
> > > We always set FIN_RCVD on it. You're right, if we only checked that on
> > > 'eof', that didn't solve much, but that wasn't necessarily intended. My
> > > original intention was to make setting of FIN_RCVD (or whatever it was
> > > originally) robust.  
> > 
> > Ok, well.  I've spotted other changes to make in the vicinity that I
> > think will make some of this easier to reason about anyway.  So I'll
> > consider your points as I rework this and other patches.
> > 
> > > > That said the conditions on which we exit / retry this loop are pretty
> > > > darn confusing.  I'll see if I can improve them.  
> > > 
> > > -- 
> > > Stefano
> 
> -- 
> Stefano
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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