Re: [PATCH v4 08/13] drm/v3d: Fix builds with CONFIG_TRANSPARENT_HUGEPAGE=n

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 16 Oct 2025 09:09:26 +0200
Loïc Molinari <loic.molinari@collabora.com> wrote:

> Hi Boris,
> 
> On 16/10/2025 07:56, Boris Brezillon wrote:
> > On Wed, 15 Oct 2025 22:41:59 +0200
> > Loïc Molinari <loic.molinari@collabora.com> wrote:
> >   
> >> On 15/10/2025 20:17, Boris Brezillon wrote:  
> >>> On Wed, 15 Oct 2025 17:30:12 +0200
> >>> Loïc Molinari <loic.molinari@collabora.com> wrote:
> >>>      
> >>>> Don't declare "super_pages" on builds with CONFIG_TRANSPARENT_HUGEPAGE
> >>>> disabled to prevent build error:
> >>>>
> >>>> ERROR: modpost: "super_pages" [drivers/gpu/drm/v3d/v3d.ko] undefined!  
> >>>
> >>> I believe this is a bug introduced by the previous commit: the
> >>> compiler probably drops any code between the
> >>> IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) check and the err label
> >>> because IS_ENABLED() evaluates to false at compile time. So I'd squash
> >>> those changes in the previous commit.  
> >>
> >> Right, it's been introduced in previous commit.
> >>  
> >>>     
> >>>>
> >>>> Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
> >>>> ---
> >>>>    drivers/gpu/drm/v3d/v3d_drv.h | 2 ++
> >>>>    drivers/gpu/drm/v3d/v3d_gem.c | 2 ++
> >>>>    2 files changed, 4 insertions(+)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/v3d/v3d_drv.h b/drivers/gpu/drm/v3d/v3d_drv.h
> >>>> index 99a39329bb85..481502104391 100644
> >>>> --- a/drivers/gpu/drm/v3d/v3d_drv.h
> >>>> +++ b/drivers/gpu/drm/v3d/v3d_drv.h
> >>>> @@ -564,7 +564,9 @@ extern const struct dma_fence_ops v3d_fence_ops;
> >>>>    struct dma_fence *v3d_fence_create(struct v3d_dev *v3d, enum v3d_queue q);
> >>>>    
> >>>>    /* v3d_gem.c */
> >>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>>    extern bool super_pages;
> >>>> +#endif
> >>>>    int v3d_gem_init(struct drm_device *dev);
> >>>>    void v3d_gem_destroy(struct drm_device *dev);
> >>>>    void v3d_reset_sms(struct v3d_dev *v3d);
> >>>> diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
> >>>> index 635ff0fabe7e..0039063eb8b2 100644
> >>>> --- a/drivers/gpu/drm/v3d/v3d_gem.c
> >>>> +++ b/drivers/gpu/drm/v3d/v3d_gem.c
> >>>> @@ -269,7 +269,9 @@ v3d_huge_mnt_init(struct v3d_dev *v3d)
> >>>>    	 * match our usecase.
> >>>>    	 */
> >>>>    
> >>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>>    	if (super_pages)
> >>>> +#endif
> >>>>    		err = drm_gem_huge_mnt_create(&v3d->drm, "within_size");  
> >>>
> >>> Why not
> >>>
> >>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>     	if (super_pages)
> >>>     		err = drm_gem_huge_mnt_create(&v3d->drm, "within_size");
> >>> #endif
> >>>
> >>> I guess
> >>>
> >>> 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && super_pages)
> >>> 		err = drm_gem_huge_mnt_create(&v3d->drm, "within_size");
> >>>
> >>> would also do, since it's likely to rely on the same optimization the
> >>> previous v3d_gemfs_init() implementation was relying on, but it's
> >>> fragile (not sure what happens when compiled with -O0).  
> >>
> >> I'll remove the #ifdef/#endif around the super_pages declaration in
> >> v3d_drv.h because it isn't necessary if super_pages is compiled out in
> >> v3d_huge_mnt_init().
> >>
> >> In v3d_huge_mnt_init(), I'd add the #ifdef before the ret variable
> >> declaration and the #endif right after the last else so that it's clear
> >> drm_notice("THP is recommended...") is called unconditionally when
> >> CONFIG_TRANSPARENT_HUGEPAGE=n, whatever the optim level. What do you think?  
> > 
> > First off, I'm not a huge fan of the following pattern
> > 
> > #if foo
> > 	if (xxxx)
> > #endif
> > 		do_something
> > 
> > which also applies to
> > 
> > #if foo
> > 	if (xxxx)
> > 		do_xxx
> > 	else if (yyy)
> > 		do_yyy
> > 	else
> > #endif
> > 		do_something
> > 
> > I'd rather have do_something duplicated in an #else section
> > like that:
> > 
> > #if foo
> > 	if (xxxx)
> > 		do_xxx
> > 	else if (yyy)
> > 		do_yyy
> > 	else
> > 		do_something
> > #else
> > 	do_something
> > #endif
> > 
> > But I'm not even seeing what the problem is here. If you do:
> > 
> > 	int err = 0;
> > 
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >      if (super_pages)
> >      	err = drm_gem_huge_mnt_create(&v3d->drm, "within_size");
> > #endif
> > 
> > 	if (v3d->drm.huge_mnt)
> > 		drm_info(&v3d->drm, "Using Transparent Hugepages\n");
> > 	else if (err)
> > 		drm_warn(&v3d->drm, "Can't use Transparent Hugepages (%d)\n", err);
> > 	else
> > 		drm_notice(&v3d->drm,
> > 			   "Transparent Hugepage support is recommended for optimal performance on this platform!\n");
> > 
> > You're guaranteed that err=0 and v3d->drm.huge_mnt=NULL when
> > CONFIG_TRANSPARENT_HUGEPAGE=n, so the "THP recommended"
> > message should be displayed unconditionally. Am I missing
> > something?  
> 
> It doesn't really matter here but I just thought it would be cleaner to 
> explicitly let just the drm_notice() because the compiler doesn't know 
> v3d->drm.huge_mnt is always NULL here and would emit a branch in 
> CONFIG_TRANSPARENT_HUGEPAGE=n builds.

Okay, well. I think it would matter if this was a hot-path, maybe? But
that's clearly not the case, this code is called once at probe time.

> I know your dislike for this 
> pattern now, so I will stick to the suggestion :)

Actually, I would probably stick to the original

	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && super_pages)
		err = drm_gem_huge_mnt_create(&v3d->drm, "within_size");

pattern since it worked so far. If -O0 is really a problem (didn't
check if it is), and v3d maintainers care about it (I doubt anyone
forces -O0 to be honest), they'll fix that in a follow-up patchset.