linux-kernel - Re: [PATCH 07/32] mm: Bring back vmalloc

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Date:   Mon, 15 May 2023 03:26:43 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

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.