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Date:   Fri, 2 Jun 2023 10:38:09 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Howells <dhowells@redhat.com>, 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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On Fri, 2 Jun 2023 13:05:14 -0400 Linus Torvalds wrote:
> Just to harp some more on this - if SPLICE_F_MORE is seen as purely a
> performance hit, with no real semantic value, and will still set
> random packet boundaries but we want big packets for all the _usual_
> cases, then I think something like "splice_end()" can be a fine
> solution regardless of exact semantics.
> 
> Alternatively, if we make it the rule that "splice_end()" is only
> called on EOF situations - so signals etc do not matter - then the
> semantics would be stable and sound fine to me too.
> 
> In that second case, I'd like to literally name it that way, and
> actually call it "splice_eof()". Because I'd like to really make it
> very clear what the semantics would be.
> 
> 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?

->splice_eof() with the proposed semantics sounds perfect for the cases
testers complained about it the past, IMHO. We can pencil that in as the
contingency plan. Actually I like these semantics so much I'm tempted to
ask David to implement it already and save users potential debugging :D

> I don't think it's conceptually any different from my suggestion of
> saying "->splice_read() can set SPLICE_F_MORE if it has more to give",
> just a different implementation that doesn't require changes on the
> splice_read() side.

Setting SPLICE_F_MORE from the input side does feel much cleaner than
guessing in splice.c. But we may end up needing the eof() callback for 
the corner cases, anyway :(