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

[David Hildenbrand (Arm)]

On 6/2/26 02:28, Andrew Morton wrote:
> On Mon, 1 Jun 2026 16:04:55 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
>> On Mon, 1 Jun 2026 18:33:25 +0100
>> Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>>
>>>
>>>
>>> FUSE might be interesting - fuse_dev_splice_read() and its ilk.
>>> Communications between the kernel and fuse server at least used to
>>> seriously want that, so that would be one place to look for unhappy
>>> userland...
>>>
>>> splice-related logics in fs/fuse/dev.c is interesting; another place
>>> like this is kernel/trace/, but I'm less familiar with that one.
>>>
>>> rostedt Cc'd (miklos already had been)
>>
>> Thanks for the Cc. The tracing ring buffer was specifically made to be used
>> by splice and the libtracefs has a lot of code to use it as well. As
>> reading the ring buffer literally swaps out the write portion with a blank
>> read portion, that portion (sub-buffer) is used to be directly fed into
>> splice, providing a zero-copy of the trace data from the write of the event
>> to going into a file.
>>
>> trace-cmd defaults to using splice to copy the tracing ring buffer directly
>> into files to avoid as much copying during live recordings as possible.
>>
>> Whatever changes we make, I would like to make sure there's no regressions
>> in performance of trace-cmd record.
> 
> Well yes, The patchset seems sensible from a quality POV.  But to make
> a decision we should first have a decent understanding of its downside
> impact.

I guess most (all?) of us ... dislike ... vmsplice(), so trying to remove it
entirely is certainly very appealing ...

> 
> I haven't seen a description of that impact in the discussion thus far.
> And that description is owed, please.
> 
> I assume a small number of specialized applications are using
> vmsplice() to great effect?  What are those applications?  What is the
> impact of this change?


I did some digging, and the kernel crypto API documents using splice/vmsplice
for zero-copy[1] and libkcapi [2].

I did not find performance numbers, how much vmsplice/splice actually gives us.
Playing with the kcapi-speed tool [3] (specifying --vmsplice vs. --sendmsg)
doesn't really reveal a big difference at least on my notebook. Not sure if the
parameters I specify are reasonable.

I don't know whether downgrading vmsplice to preadv2/pwritev2 would perform
significantly worse than sendmsg ... and I don't know what the default would
usually be (default to vmsplice or sendmsg). I might try finding some time to
play with it more, but I doubt it, so if anybody else has time ... :)


I'll note that we have a bunch of selftests (mostly around COW handling) that
rely on vmsplice to test R/O pinning behavior. For R/W pinning, we can use
iouring fixed buffers easily. The only alternative for R/O pinning is using the
gup_test infrastructure that needs to be compiled into the kernel, unfortunately ...

So we'll have to adjust some tests there to use a different interface. I'm sure
I can find someone to work on that once this change here landed and doesn't have
to be yanked immediately again.


[1] https://www.kernel.org/doc/html/latest/crypto/userspace-if.html
[2] https://github.com/smuellerDD/libkcapi/blob/master/lib/kcapi-kernel-if.c
[3] https://github.com/smuellerDD/libkcapi/tree/master/speed-test

-- 
Cheers,

David