linux-kernel - Re: copy on write for splice() from file to pipe?

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Date:   Fri, 10 Feb 2023 14:35:11 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Ming Lei <ming.lei@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Stefan Metzmacher <metze@...ba.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API Mailing List <linux-api@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Samba Technical <samba-technical@...ts.samba.org>
Subject: Re: copy on write for splice() from file to pipe?

On Fri, Feb 10, 2023 at 2:26 PM Jens Axboe <axboe@...nel.dk> wrote:
> >
> > (I actually suspect that /dev/zero no longer works as a splice source,
> > since we disabled the whole "fall back to regular IO" that Christoph
> > did in 36e2c7421f02 "fs: don't allow splice read/write without
> > explicit ops").
>
> Yet another one... Since it has a read_iter, should be fixable with just
> adding the generic splice_read.

I actually very consciously did *not* want to add cases of
generic_splice_read() "just because we can".

I've been on a "let's minimize the reach of splice" thing for a while.
I really loved Christoph's patches, even if I may not have been hugely
vocal about it. His getting rid of set/get_fs() got rid of a *lot* of
splice pain.

And rather than try to make everything work with splice that used to
work just because it fell back on read/write, I was waiting for actual
regression reports.

Even when splice fails, a lot of user space then falls back on
read/write, and unless there is some really fundamental reason not to,
I think that's always the right thing to do.

So we do have a number of "add splice_write/splice_read" commits, but
they are hopefully all the result of people actually noticing
breakage.

You can do

     git log --grep=36e2c7421f02

to see at least some of them, and I really don't want to see them
without a "Reported-by" and an actual issue.

Exactly because I'm not all that enamoured with splice any more.

                 Linus