On Thu, Jun 04, 2026 at 08:58:33AM -0700, Linus Torvalds wrote:

On Thu, 4 Jun 2026 at 08:53, Willy Tarreau [off-list ref] wrote:

quoted

quoted

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