Re: [PATCH 07/32] mm: Bring back vmalloc_exec

Kent Overstreet
2023-05-15T07:27:03Z

On Mon, May 15, 2023 at 12:13:43AM -0700, Eric Biggers wrote:
> Sure, given that this is an optimization problem with a very small scope
> (decoding 6 fields from a bitstream), I was hoping for something easier and
> faster to iterate on than setting up a full kernel + bcachefs test environment
> and reverse engineering 500 lines of shell script. But sure, I can look into
> that when I have a chance.

If you were actually wanting to help, that repository is the tool I use
for kernel development and testing - it's got documentation.

It builds a kernel, boots a VM and runs a test in about 15 seconds, no
need for lifting that code out to userspace.

> > Your approach wasn't any faster than the existing C version.
>
> Well, it's your implementation of what you thought was "my approach". It
> doesn't quite match what I had suggested. As I mentioned in my last email, it's
> also unclear that your new code is ever actually executed, since you made it
> conditional on all fields being byte-aligned...

Eric, I'm not an idiot, that was one of the first things I checked. No
unaligned bkey formats were generated in my tests.

The latest iteration of your approach that I looked at compiled to ~250
bytes of code, vs. ~50 bytes for the dynamically generated unpack
functions. I'm sure it's possible to shave a bit off with some more
work, but looking at the generated code it's clear it's not going to be
competitive.