linux-kernel - Re: [PATCH] x86/mm: drop superfluous initialization

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Message-ID: <alpine.DEB.2.21.2009291034160.17656@felia>
Date:   Tue, 29 Sep 2020 10:42:15 +0200 (CEST)
From:   Lukas Bulwahn <lukas.bulwahn@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH] x86/mm: drop superfluous initialization



On Mon, 28 Sep 2020, Dave Hansen wrote:

> On 9/28/20 3:00 AM, Lukas Bulwahn wrote:
> > diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> > index c7a47603537f..5632f02146ca 100644
> > --- a/arch/x86/mm/init.c
> > +++ b/arch/x86/mm/init.c
> > @@ -609,7 +609,7 @@ static void __init memory_map_top_down(unsigned long map_start,
> >  	step_size = PMD_SIZE;
> >  	max_pfn_mapped = 0; /* will get exact value next */
> >  	min_pfn_mapped = real_end >> PAGE_SHIFT;
> > -	last_start = start = real_end;
> > +	last_start = real_end;
> 
> Thanks for finding this.
> 
> This becomes even more obviously correct if we just move the 'start'
> declaration into the while() loop.  If we do that, it puts the three
> assignment locations right next to the definition, and its trivial to
> spot that the initialization was not missed:
> 
>         while (last_start > map_start) {
> 		unsigned long start;
> 
>                 if (last_start > step_size) {
>                         start = round_down(last_start - 1, step_size);
>                         if (start < map_start)
>                                 start = map_start;
>                 } else
>                         start = map_start;
> 		...
>

Agree, this point is simply a question of style:

Shall local variables be defined as "local" as possible or simply 
consistently at the beginning of each function?

If there are no strong opinions of style, I would just keep this patch 
as-is.

> Either way, your patch looks correct to me:
> 
> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>
> 

Thanks for the Ack.

Lukas