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

[Vlastimil Babka]

On 10/11/21 19:08, Marco Elver wrote:
> On Mon, 11 Oct 2021 at 19:02, Vlastimil Babka <vbabka@xxxxxxx> 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.

Ah, the file only includes stackdepot.h in a #ifdef CONFIG_DRM_DEBUG_MM
section so I will keep the #ifdef here for a minimal change, unless
requested otherwise.