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

From: Willy Tarreau <[email-masked]>
Date: Thu, 4 Jun 2026 18:15:41 +0200

On Thu, Jun 04, 2026 at 08:58:33AM -0700, Linus Torvalds wrote:
> On Thu, 4 Jun 2026 at 08:53, Willy Tarreau <[email-masked]> wrote:
> >
> > > It looks like you're actually doing exactly the thing that I thought
> > > was crazy and wouldn't even work reliably: you change the
> > > common_response[] contents dynamically *after* the vmsplice, and
> > > depend on the fact that changing it in user space changes the buffer
> > > in the pipe too.
> >
> > No no, it's definitely not doing that (or it's a bug, but it's not
> > supposed to happen). I'm perfectly aware that one must definitely not
> > do that, and it's a guarantee the user of vmsplice() must provide.
> 
> Whew, good.
> 
> In that case, can you just try the vmsplice patch series (Christian
> already found a bug, but I don't think it will necessarily matter in
> practice - famous last words) and that test patch of mine, and see if
> it all (a) works for you and (b) if you have any numbers for
> performance that would be *great*.

Yes I wanted to do that and noted it on my todo list yesterday when
noticing the ongoing discussion. Just been super busy with yesterday's
by-yearly release ;-) But at least I wanted to share quick feedback in
this thread about existing uses.

> There aren't many obvious splice users out there, and even if they
> were to exist they are typically specialized enough that you have to
> have a real use case to then tell if the patches make a difference in
> real life or not.

I totally agree, that's why I want to share some feedback. I remember
years ago when splice() was broken in 2.6.25, there were so few users
that I was the one reporting an API issue to Eric who addressed it
early by lack of users. And I even consider that due to the very few
users, it's even acceptable to slightly change the way to use it if
it can provide extra guarantees (like requiring a capability to access
non-anonymous pages for example). It should not break that many apps,
and as long as they can preserve their essential benefits, I think
most will be OK to adapt.

> So you testing that thing would seem to be a great first test of
> whether any of this is realistic..

Absolutely! 

Willy