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

From: Kent Overstreet
Date: Tue May 09 2023 - 17:54:36 EST

On Tue, May 09, 2023 at 02:43:19PM -0700, Darrick J. Wong wrote:
> On Tue, May 09, 2023 at 02:12:41PM -0700, Lorenzo Stoakes wrote:
> > On Tue, May 09, 2023 at 01:46:09PM -0700, Christoph Hellwig wrote:
> > > On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote:
> > > > From: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > > >
> > > > This is needed for bcachefs, which dynamically generates per-btree node
> > > > unpack functions.
> > >
> > > No, we will never add back a way for random code allocating executable
> > > memory in kernel space.
> >
> > Yeah I think I glossed over this aspect a bit as it looks ostensibly like simply
> > reinstating a helper function because the code is now used in more than one
> > place (at lsf/mm so a little distracted :)
> >
> > But it being exported is a problem. Perhaps there's another way of acheving the
> > same aim without having to do so?
> I already trolled Kent with this on IRC, but for the parts of bcachefs
> that want better assembly code than whatever gcc generates from the C
> source, could you compile code to BPF and then let the BPF JIT engines
> turn that into machine code for you?

It's an intriguing idea, but it'd be a _lot_ of work and this is old
code that's never had a single bug - I'm not in a hurry to rewrite it.

And there would still be the issue that we've still got lots of little
unpack functions that go with other tables; we can't just burn a full
page per unpack function, that would waste way too much memory, and if
we put them together then we're stuck writing a whole nother allocator
- nope, and then we're also mucking with the memory layout of the data
structures used in the very hottest paths in the filesystem - I'm very
wary of introducing performance regressions there.

I think it'd be much more practical to find some way of making
vmalloc_exec() more palatable. What are the exact concerns?