Re: [PATCH 0/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2

On Wed, 3 Jun 2026 at 06:40, Christian Brauner <[email protected]> wrote:
>
> Prior to the change add_to_pipe() returns -EAGAIN the moment the pipe is
> full. So iter_to_pipe stops and returns a partial count capped at pipe
> capacity. For a 128K buffer over a 64K pipe the first call returns 64K,
> the test drains it, call 2 returns the remaining 64K. Done.
>
> After this change do_writev(... flags & SPLICE_F_NONBLOCK ? RWF_NOWAIT :
> 0) then calls pipe_write which does not stop when the pipe fills. It
> blocks until the entire iovec is consumed.
>
> I kinda think we need to preserve similar semantics.

Ack. We definitely do need to keep the old semantics.

Looking at the patch again, I think it's that

    (flags & SPLICE_F_NONBLOCK) ? RWF_NOWAIT : 0

thing that is broken. I think splice_to_pipe is *always* nowait - but
has the special conditional _initial_ wait.

So I think the RWF_NOWAIT should be unconditional to the do_writev(),
and instead the code should do something like

        ret = wait_for_space(pipe, flags);
        if (!ret) do_writev(...RWF_NOWAIT);

but admittedly I did not think very much about the details, so I might
miss something.

Which also then probably measn that we should just keep the legacy
wrapper in fs/splice.c and we'd just need to make do_writev() and
do_readv() non-static.

Because I'd rather keep wait_for_space() internal to splice (or
alternatively we'd move it to pipe.c, rename it to
"pipe_wait_for_space()", and change the 'flags' argument to be a
boolean to not make it use that splice-specific flags etc).

            Linus