linux-kernel - Re: Bug in short splice to socket?

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Date:   Fri, 2 Jun 2023 09:39:29 -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 08:11:47 -0400 Linus Torvalds wrote:
> If then some *real* load ends up showing a regression, we may just be
> screwed. Our current behavior may be buggy, but we have the rule that
> once user space depends on kernel bugs, they become features pretty
> much by definition, however much we might dislike it.
> 
> At that point, we'll have to see what we can do - if anything.

Can we have a provisional plan of how we'll fix it if someone does
complain? We can't just revert David's work, and if none of the
solutions are appealing - socket implementations may be left holding
the bag.

I dislike the magic zero sends, and I think you do, too. In case of TLS
its unclear whether we should generate an empty record (like UDP would).

Can we add an optional splice_end / short_splice / splice_underflow /
splice_I_did_not_mean_to_set_more_on_the_previous_call_sorry callback
to struct file_operations?