netdev - Re: Bug in short splice to socket?

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Message-ID: <20230602103809.1510cbef@kernel.org>
Date: Fri, 2 Jun 2023 10:38:09 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Howells <dhowells@...hat.com>, netdev@...r.kernel.org, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, David Ahern <dsahern@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, Chuck Lever
 <chuck.lever@...cle.com>, Boris Pismenny <borisp@...dia.com>, John
 Fastabend <john.fastabend@...il.com>, Christoph Hellwig <hch@...radead.org>
Subject: Re: Bug in short splice to socket?

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 :(