Re: Bug in short splice to socket?

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jakub Kicinski <kuba@kernel.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?
Date: Fri, 2 Jun 2023 13:05:14 -0400	[thread overview]
Message-ID: <CAHk-=wh=V579PDYvkpnTobCLGczbgxpMgGmmhqiTyE34Cpi5Gg@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whgpCNzmQfTAUY7D8P6t9TgzoLx9Uauu7YGQpgZtg-SYg@mail.gmail.com>

On Fri, Jun 2, 2023 at 12:53 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> And no, I don't think "splice_end()" can be that exact semantics -
> even if it's simple - exactly because splice() is an interruptible
> operation, so the "end" of a splice() is simply not a stable thing.

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?

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.

                 Linus