Re: [PATCH 0/3] Type aware module allocator

From: Song Liu
Date: Sun May 28 2023 - 01:58:58 EST

On Sat, May 27, 2023 at 12:04 AM Kent Overstreet
<kent.overstreet@xxxxxxxxx> wrote:
> On Thu, May 25, 2023 at 10:15:26PM -0700, Song Liu wrote:
> > This set implements the second part of module type aware allocator
> > (module_alloc_type), which was discussed in [1]. This part contains the
> > interface of the new allocator, as well as changes in x86 code to use the
> > new allocator (modules, BPF, ftrace, kprobe).
> >
> > The set does not contain a binpack allocator to enable sharing huge pages
> > among different allocations. But this set defines the interface used by
> > the binpack allocator. [2] has some discussion on different options of the
> > binpack allocator.
> I'm afraid the more I look at this patchset the more it appears to be
> complete nonsense.

I don't think it is nonsense, as you clearly got most of the points here. :)

> The exposed interface appears to be both for specifying architecture
> dependent options (which should be hidden inside the allocator
> internals!) _and_ a general purpose interface for module/bpf/kprobes -
> but it's not very clear, and the rational appears to be completely
> missing.

The rationale is to have something to replace module_alloc(). Therefore,
it needs to handle architecture specific requirements, and provide
interface to all current users of module_alloc(). It may look a little weird
at the moment, because the actual allocator logic is very thin. But that's
where we will plug in the next step of the work.

> I think this needs to back to the drawing board and we need something
> simpler just targeted at executable memory; architecture specific
> options should definitely _not_ be part of the exposed interface.

I don't think we are exposing architecture specific options to users.
Some layer need to handle arch specifics. If the new allocator is
built on top of module_alloc, module_alloc is handling that. If the new
allocator is to replace module_alloc, it needs to handle arch specifics.

> The memory protection interface also needs to go, we've got a better
> interface to model after (text_poke(), although that code needs work
> too!). And the instruction fill features need a thorough justification
> if they're to be included.

I guess the first step to use text_poke() is to make it available on all
archs? That doesn't seem easy to me.