linux-kernel - Re: [PATCH] lib/stackdepot: allow optional init and stack

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Message-ID: <f4f9c5b8-7bff-4d5d-8768-5e58ee1cc907@suse.cz>
Date:   Tue, 12 Oct 2021 10:31:43 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, kasan-dev@...glegroups.com,
        Vijayanand Jitta <vjitta@...eaurora.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Oliver Glitta <glittao@...il.com>,
        Imran Khan <imran.f.khan@...cle.com>
Subject: Re: [PATCH] lib/stackdepot: allow optional init and stack_table
 allocation by kvmalloc()

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

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.