Re: [PATCH 1/3] module: Introduce module_alloc_type

From: Song Liu
Date: Sat May 27 2023 - 02:00:47 EST

On Fri, May 26, 2023 at 8:20 PM Kent Overstreet
<kent.overstreet@xxxxxxxxx> wrote:
> > > void jitalloc_update(void *dst, void *src, size_t len)
> > > {
> > > if (text_poke_available) {
> > > text_poke(...);
> > > } else {
> > > unprotect();
> > > memcpy();
> > > protect();
> > > }
> > > }
> >
> > I think this doesn't work for sub page allocation?
> Perhaps I elided too much - it does if you add a single lock. You can't
> do that if it's not a common helper.

Actually, sub page allocation is not the problem. The problem is
with unprotect() and protect(), as they require global TLB flush.

> > At the end of all this, we will have modules running from huge pages, data
> > and text. It will give significant performance benefit when some key driver
> > cannot be compiled into the kernel.
> Yeah, I've seen the numbers for the perf impact of running as a module
> due to the extra TLB overhead - but Mike's recent data was showing that
> this doesn't matter nearly as much as data as it does for text.
> > > Also - module_memory_fill_type(), module_memory_invalidate_type() - I
> > > know these are for BPF, but could you explain why we need them in the
> > > external interface here? Could they perhaps be small helpers in the bpf
> > > code that use something like jitalloc_update()?
> >
> > These are used by all users, not just BPF. 1/3 uses them in
> > kernel/module/main.c. I didn't use them in 3/3 as it is arch code, but I can
> > use them instead of text_poke_* (and that is probably better code style).
> > As I am writing the email, I think I should use it in ftrace.c (2/3) to avoid
> > the __weak function.
> Ok. Could we make it clearer why they're needed and what they're for?
> I know bpf fills in invalid instructions initially; I didn't read
> through enough code to find the "why", and let's document that to save
> other people the same effort.
> And do they really need to be callbacks in mod_alloc_params...?

If we want to call them from common code, they will either be callback
or some __weak functions.

> Do we need the other things in mod_alloc_params/vmalloc_params?
> - your granularity field says it's for specifying PAGE/PMD size: we
> definitely do not want that. We've had way too much code along the
> lines of "implement hugepages for x", we need to stop doing that.
> It's an internal mm/ detail.

We don't need "granularity" yet. We will need them with the binpack

> - vmalloc_params: we need gfp_t and pgprot_t, but those should be
> normal arguments. start/end/vm_flags? They seem like historical
> module baggage, can we get rid of them?

All these fields are needed by some of the module_alloc()s for different
archs. I am not an expert of all the archs, so I cannot say whether some
of them are not needed.
> > I don't really think module code is very messy at the moment. If
> > necessary, we can put module_alloc_type related code in a
> > separate file.
> Hey, it's been organized since I last looked at it :) But I tihink this
> belongs in mm/, not module code.

I don't have a strong preference for this (at the moment). If we agree
it belongs to mm/, we sure can move it.