Re: [PATCH] mm/slub: quiet the clang warning with -Wunused-function enabled

[Suren Baghdasaryan]

On Wed, Jul 10, 2024 at 8:02 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Wed, Jul 10, 2024 at 04:03:33AM +0100, Matthew Wilcox wrote:
> > On Wed, Jul 10, 2024 at 10:54:18AM +0800, sxwjean@xxxxxx wrote:
> > > From: Xiongwei Song <xiongwei.song@xxxxxxxxx>
> > >
> > > The only user of prepare_slab_obj_exts_hook() is
> > > alloc_tagging_slab_alloc_hook(), which can build with
> > > CONFIG_MEM_ALLOC_PROFILING enabled. So, the warning was triggerred
> > > when disabling CONFIG_MEM_ALLOC_PROFILING. Let's add "__maybe_unused"
> > > for prepare_slab_obj_exts_hook().
> >
> > Perhaps instead clang can be fixed to match gcc's behaviour?
>
> Clang only differs from GCC on warning for unused static inline functions in .c
> files, not .h files. The kernel already handles this in
> include/linux/compiler_types.h but it disables this workaround for W=1 to catch
> unused functions like this as a result of commit 6863f5643dd7 ("kbuild: allow
> Clang to find unused static inline functions for W=1 build"):
>
> /*
>  * GCC does not warn about unused static inline functions for -Wunused-function.
>  * Suppress the warning in clang as well by using __maybe_unused, but enable it
>  * for W=1 build. This will allow clang to find unused functions. Remove the
>  * __inline_maybe_unused entirely after fixing most of -Wunused-function warnings.
>  */
> #ifdef KBUILD_EXTRA_WARN1
> #define __inline_maybe_unused
> #else
> #define __inline_maybe_unused __maybe_unused
> #endif
>
> So I don't really think there is much for clang to do here and I think having
> the ability to find unused static inline functions in .c files is useful (you
> might disagree, perhaps a revert could still be discussed). I guess
> IS_ENABLED() can't be used there, so it seems like either taking this patch,
> ignoring the warning, or refactoring the code in some other way are the only
> options I see.

I think this is the consequence of the recent refactoring I've done in
https://lore.kernel.org/all/20240704135941.1145038-1-surenb@xxxxxxxxxx/.
There should be a cleaner way to fix this. I'll post it later today or
tomorrow morning.
Thanks,
Suren.

>
> > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > Closes: https://lore.kernel.org/oe-kbuild-all/202407050845.zNONqauD-lkp@xxxxxxxxx/
> > > Signed-off-by: Xiongwei Song <xiongwei.song@xxxxxxxxx>
> > > ---
> > >  mm/slub.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index ce39544acf7c..2e26f20759c0 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -2027,7 +2027,7 @@ static inline bool need_slab_obj_ext(void)
> > >     return false;
> > >  }
> > >
> > > -static inline struct slabobj_ext *
> > > +static inline struct slabobj_ext * __maybe_unused
> > >  prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> > >  {
> > >     struct slab *slab;
> > > @@ -2068,7 +2068,7 @@ static inline bool need_slab_obj_ext(void)
> > >     return false;
> > >  }
> > >
> > > -static inline struct slabobj_ext *
> > > +static inline struct slabobj_ext * __maybe_unused
> > >  prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> > >  {
> > >     return NULL;
> > > --
> > > 2.34.1
> > >
> > >