linux-kernel - Re: [PATCH v2] arm64/mm: fix variable 'tag' set but not used

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Message-ID: <1564676434.11067.46.camel@lca.pw>
Date:   Thu, 01 Aug 2019 12:20:34 -0400
From:   Qian Cai <cai@....pw>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     catalin.marinas@....com, will@...nel.org, andreyknvl@...gle.com,
        aryabinin@...tuozzo.com, glider@...gle.com, dvyukov@...gle.com,
        linux-arm-kernel@...ts.infradead.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64/mm: fix variable 'tag' set but not used

On Thu, 2019-08-01 at 09:00 -0700, Matthew Wilcox wrote:
> On Thu, Aug 01, 2019 at 10:47:05AM -0400, Qian Cai wrote:
> 
> Given this:
> 
> > -#define __tag_set(addr, tag)	(addr)
> > +static inline const void *__tag_set(const void *addr, u8 tag)
> > +{
> > +	return addr;
> > +}
> > +
> >  #define __tag_reset(addr)	(addr)
> >  #define __tag_get(addr)		0
> >  #endif
> > @@ -301,8 +305,8 @@ static inline void *phys_to_virt(phys_addr_t x)
> >  #define page_to_virt(page)	({					
> > \
> >  	unsigned long __addr =						
> > \
> >  		((__page_to_voff(page)) | PAGE_OFFSET);			
> > \
> > -	unsigned long __addr_tag =					\
> > -		 __tag_set(__addr, page_kasan_tag(page));		\
> > +	const void *__addr_tag =					\
> > +		__tag_set((void *)__addr, page_kasan_tag(page));	\
> >  	((void *)__addr_tag);						
> > \
> >  })
> 
> Can't you simplify that macro to:
> 
>  #define page_to_virt(page)	({					\
>  	unsigned long __addr =						
> \
>  		((__page_to_voff(page)) | PAGE_OFFSET);			
> \
> -	unsigned long __addr_tag =					\
> -		 __tag_set(__addr, page_kasan_tag(page));		\
> -	((void *)__addr_tag);						
> \
> +	__tag_set((void *)__addr, page_kasan_tag(page));		\
>  })

It still need a cast or lowmem_page_address() will complain of a discarded
"const". It might be a bit harder to read when adding a cast as in,

((void *)__tag_set((void *)__addr, page_kasan_tag(page)));

But, that feel like more of a followup patch for me if ever needed.