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From:   David Howells <dhowells@redhat.com>
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     dhowells@redhat.com, Jakub Kicinski <kuba@kernel.org>,
        netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Chuck Lever <chuck.lever@oracle.com>,
        Boris Pismenny <borisp@nvidia.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Christoph Hellwig <hch@infradead.org>
Subject: Re: Bug in short splice to socket?
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Date:   Fri, 02 Jun 2023 21:38:40 +0100
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Linus Torvalds <torvalds@linux-foundation.org> wrote:

> So a "splice_eof()" sounds fine to me, and we'd make the semantics be
> the current behavior:
> 
>  - splice() sets SPLICE_F_MORE if 'len > read_len'
> 
>  - splice() _clears_ SPLICE_F_MORE if we have hit 'len'
> 
>  - splice always sets SPLICE_F_MORE if it was passed by the user
> 
> BUT with the small new 'splice_eof()' rule that:
> 
>  - if the user did *not* set SPLICE_F_MORE *and* we didn't hit that
> "use all of len" case that cleared SPLICE_F_MORE, *and* we did a
> "->splice_in()" that returned EOF (ie zero), *then* we will also do
> that ->splice_eof() call.
> 
> The above sounds like "stable and possibly useful semantics" to me. It
> would just have to be documented.
> 
> Is that what people want?

That's easier to implement, I think.  That's basically what I was trying to
achieve by sending a zero-length actor call, but this is a cleaner way of
doing it, particularly if it's added as a socket op next to ->sendmsg().

Otherwise I have to build up the input side to try and tell me in advance
whether it thinks we hit an EOF/hole/whatever condition.  The problem is that,
as previously mentioned, it doesn't work for all circumstances - seqfile,
pipes, sockets for instance.

Take the following scenario for example: I could read from a TCP socket,
filling up the pipe-buffer, but not with sufficient data to fulfill the
operation.  Say I drain the TCP socket, but it's still open, so might produce
more data.  I then call the actor, which passes all the data to sendmsg() with
MSG_SPLICE_PAGES and MSG_MORE and clears the buffer.  I then go round again,
but in the meantime, the source socket got shut down with no further data
available and do_splice_to() returns 0.

There's no way to predict this, so having a ->splice_eof() call would handle
this situation.

David