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

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jun 19, 2023 at 06:47:17AM -0400, Kent Overstreet wrote:
> On Mon, Jun 19, 2023 at 10:19:00AM +0100, Mark Rutland wrote:
> > On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote:
> > > From: Kent Overstreet <kent.overstreet@gmail.com>
> > > 
> > > This is needed for bcachefs, which dynamically generates per-btree node
> > > unpack functions.
> > 
> > Much like Kees and Andy, I have concerns with adding new code generators to the
> > kernel. Even ignoring the actual code generation, there are a bunch of subtle
> > ordering/maintenance/synchronization concerns across architectures, and we
> > already have a fair amount of pain with the existing cases.
> 
> Look, jits are just not that unusual. I'm not going to be responding to
> vague concerns that don't have any actual engineering rational.

Sorry, but I do have an engineering rationale here: I want to make sure that
this actually works, on architectures that I care about, and will be
maintanable long-term.

We've had a bunch of problems with other JITs ranging from JIT-local "we got
the encoding wrong" to major kernel infrastructure changes like tasks RCU rude
synchronization. I'm trying to figure out whether any of those are likely to
apply and/or whether we should be refactoring other infrastructure for use here
(e.g. the factoring the acutal instruction generation from arch code, or
perhaps reusing eBPF so this can be arch-neutral).

I appreciate that's not clear from my initial mail, but please don't jump
straight to assuming I'm adversarial here.

> > Can you share more detail on how you want to use this?
> > 
> > From a quick scan of your gitweb for the bcachefs-for-upstream branch I
> > couldn't spot the relevant patches.
> 
> I've already written extensively in this thread.

Sorry, I hadn't seen that.

For the benefit of others, the codegen is at:

  https://lore.kernel.org/lkml/ZFq7JhrhyrMTNfd%2F@moria.home.lan/
  https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/bkey.c#n727

... and the rationale is at:

  https://lore.kernel.org/lkml/ZF6HHRDeUWLNtuL7@moria.home.lan/

One thing I note mmediately is that HAVE_BCACHEFS_COMPILED_UNPACK seems to be
x86-only. If this is important, that'll need some rework to either be
arch-neutral or allow for arch-specific implementations.

Thanks,
Mark.