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

On Mon, 1 Jun 2026 at 09:42, Christian Brauner <[email protected]> wrote:
>
> Applied to the vfs-7.2.vmsplice branch of the vfs/vfs.git tree.

Btw, if people want to work further on this - assuming we don't get
any huge screams of pain from having effectively gotten rid of
vmsplice() - I don't think it would hurt to look at limiting the
"regular" splice() too.

We already have the code to just turn it into a pure copy on the
"splice to pipe" case: copy_splice_read(). In many ways it would be
*lovely* to just always force that path.

We already do that explicitly for DAX and O_DIRECT, but we made a lot
of special files do it implicitly too, so quite a lot of the splice
reading cases already use that "just read() into a kernel space
buffer" model for splicing.

It would be interesting to hear who would even notice if we just
always used that copy case, and made "f_op->splice_read" never trigger
at all.

And it turns out that the only thing that ever uses
"f_op->splice_write" is splice_to_socket. Which was actually the
problematic buggy case.

Everybody else pretty much seems to just use iter_file_splice_write(),
which does the "emulate it with just a write from kernel buffers".

So *if* we get rid of f_op->splice_read, we do leave the case that
really caused problems, but nobody will ever care. Because once splice
only deals with private buffers that can't be shared with anything
else, a f_op->splice_write() that gets things wrong is pretty much a
non-event.

(We'd have to look at 'tee()' too: I don't think anybody really uses
it, but it does do the "no copy linking" by just incrementing
refcounts on the pipe buffers. So to really protect against
splice_write users messing up, that should do copies too, but as long
as it's all "private ephemeral buffers" that get their refcounts
updated, I don't think anybody *really* cares)

TLDR: maybe we could ghet rid of "f_op->splice_read". *That* would be
a big simplification.

                Linus