Re: [PATCH] lib/stackdepot: allow optional init and stack_table allocation by kvmalloc()

[Marco Elver <elver@google.com>]

On Mon, 11 Oct 2021 at 19:02, Vlastimil Babka <vbabka@suse.cz> wrote:
[...]
> > On the other hand, the lazy initialization mode you're introducing
> > requires an explicit stack_depot_init() call somewhere and isn't as
> > straightforward as before.
> >
> > Not sure what is best. My intuition tells me STACKDEPOT_LAZY_INIT would
> > be safer as it's a deliberate opt-in to the lazy initialization
> > behaviour.
>
> I think it should be fine with ALWAYS_INIT. There are not many stackdepot
> users being added, and anyone developing a new one will very quickly find
> out if they forget to call stack_depot_init()?

I think that's fine.

> > Preferences?
> >
> > [...]
> >> --- a/drivers/gpu/drm/drm_mm.c
> >> +++ b/drivers/gpu/drm/drm_mm.c
> >> @@ -980,6 +980,10 @@ void drm_mm_init(struct drm_mm *mm, u64 start, u64 size)
> >>      add_hole(&mm->head_node);
> >>
> >>      mm->scan_active = 0;
> >> +
> >> +#ifdef CONFIG_DRM_DEBUG_MM
> >> +    stack_depot_init();
> >> +#endif
> >
> > DRM_DEBUG_MM implies STACKDEPOT. Not sure what is more readable to drm
> > maintainers, but perhaps it'd be nicer to avoid the #ifdef here, and
> > instead just keep the no-op version of stack_depot_init() in
> > <linux/stackdepot.h>. I don't have a strong preference.
>
> Hm, but in case STACKDEPOT is also selected by something else (e.g.
> CONFIG_PAGE_OWNER) which uses lazy init but isn't enabled on boot, then
> without #ifdef CONFIG_DRM_DEBUG_MM above, this code would call a
> stack_depot_init() (that's not a no-op) even in case it's not going to be
> using it, so not what we want to achieve.
> But it could be changed to use IS_ENABLED() if that's preferred by DRM folks.

You're right -- but I'll leave this to DRM folks.

> BTW it's possible that there won't be any DRM review because this failed to
> apply:
> https://patchwork.freedesktop.org/series/95549/
> DRM folks, any hint how to indicate that the base was next-20211001?
>
[...]
> > +#ifdef CONFIG_STACKDEPOT_ALWAYS_INIT
> > +static inline int stack_depot_early_init(void)       { return stack_depot_init(); }
> > +#else
> > +static inline int stack_depot_early_init(void)       { return 0; }
> > +#endif       /* CONFIG_STACKDEPOT_ALWAYS_INIT */
>
> We could, but it's a wrapper made for only a single caller...
>
> >>  #endif
> >> diff --git a/init/main.c b/init/main.c
> >> index ee4d3e1b3eb9..b6a5833d98f5 100644
> >> --- a/init/main.c
> >> +++ b/init/main.c
> >> @@ -844,7 +844,8 @@ static void __init mm_init(void)
> >>      init_mem_debugging_and_hardening();
> >>      kfence_alloc_pool();
> >>      report_meminit();
> >> -    stack_depot_init();
> >> +    if (IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT))
> >> +            stack_depot_init();
> >
> > I'd push the decision of when to call this into <linux/stackdepot.h> via
> > wrapper stack_depot_early_init().
>
> No strong preferrences, if you think it's worth it.

All the other *init() functions seem to follow the same idiom as there
are barely any IS_ENABLED() in init/main.c.  So I think it's just
about being consistent with the rest.

Thanks,
-- Marco