Re: [RFC PATCH 0/4] splice: Fix corruption in data spliced to pipe

[Matt Whitlock]

On Thursday, 29 June 2023 13:56:04 EDT, Linus Torvalds wrote:
> On Thu, 29 Jun 2023 at 08:55, David Howells <dhowells@xxxxxxxxxx> wrote:
> > Matt Whitlock, Matthew Wilcox and Dave Chinner are of the 
> > opinion that data
> > in the pipe must not be seen to change and that if it does, this is a bug.
>
> I'm not convinced.
>
> The whole *point* of vmsplice (and splicing from a file) is the zero-copy.
>
> If you don't want the zero-copy, then you should use just "write()".

If you want zero copies, then call splice() *with* SPLICE_F_MOVE.

If you want one copy (kernel-to-kernel), then call splice() *without* 
SPLICE_F_MOVE.

If you want two copies (kernel-to-user + user-to-kernel), call read() and 
write().

I don't know why SPLICE_F_MOVE is being ignored in this thread. Sure, maybe 
the way it has historically been implemented was only relevant when the 
input FD is a pipe, but that's not what the man page implies. You have the 
opportunity to make it actually do what it says on the tin.