linux-kernel - Re: [PATCH 1/3] mm/slub: Only define kmalloc_large_node

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Date:   Wed, 24 May 2017 13:36:21 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Matthias Kaehlcke <mka@...omium.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH 1/3] mm/slub: Only define kmalloc_large_node_hook() for
 NUMA systems

On Tue, 23 May 2017, Matthias Kaehlcke wrote:

> > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> > index de179993e039..e1895ce6fa1b 100644
> > --- a/include/linux/compiler-clang.h
> > +++ b/include/linux/compiler-clang.h
> > @@ -15,3 +15,8 @@
> >   * with any version that can compile the kernel
> >   */
> >  #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
> > +
> > +#ifdef inline
> > +#undef inline
> > +#define inline __attribute__((unused))
> > +#endif
> 
> Thanks for the suggestion!
> 
> Nothing breaks and the warnings are silenced. It seems we could use
> this if there is a stong opposition against having warnings on unused
> static inline functions in .c files.
> 

It would be slightly different, it would be:

#define inline inline __attribute__((unused))

to still inline the functions, I was just seeing if there was anything 
else that clang was warning about that was unrelated to a function's 
inlining.

> Still I am not convinced that gcc's behavior is preferable in this
> case. True, it saves us from adding a bunch of __maybe_unused or
> #ifdefs, on the other hand the warning is a useful tool to spot truly
> unused code. So far about 50% of the warnings I looked into fall into
> this category.
> 

I think gcc's behavior is a result of how it does preprocessing and is a 
clearly defined and long-standing semantic given in the gcc manual 
regarding -Wunused-function.

#define IS_PAGE_ALIGNED(__size)	(!(__size & ((size_t)PAGE_SIZE - 1)))
static inline int is_page_aligned(size_t size)
{
	return !(size & ((size_t)PAGE_SIZE - 1));
}

Gcc will not warn about either of these being unused, regardless of -Wall, 
-Wunused-function, or -pedantic.  Clang, correct me if I'm wrong, will 
only warn about is_page_aligned().

So the argument could be made that one of the additional benefits of 
static inline functions is that a subset of compilers, heavily in the 
minority, will detect whether it's unused and we'll get patches that 
remove them.  Functionally, it would only result in LOC reduction.  But, 
isn't adding #ifdef's to silence the warning just adding more LOC?

I have no preference either way, I think it would be up to the person who 
is maintaining the code and has to deal with the patches.