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Date:   Wed, 20 May 2020 16:46:28 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Mike Rapoport <rppt@linux.ibm.com>
Cc:     kbuild test robot <lkp@intel.com>, kbuild-all@lists.01.org,
        linux-kernel@vger.kernel.org,
        Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: arch/sparc/mm/srmmu.c:300:9: error: variable 'pud' set but not
 used
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        <20200520132005.GM1059226@linux.ibm.com>
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On Wed, 20 May 2020 16:20:05 +0300 Mike Rapoport <rppt@linux.ibm.com> wrote:

> The kbuild test robot reported the following warning:
> 
> arch/sparc/mm/srmmu.c: In function 'srmmu_nocache_init':
> >> arch/sparc/mm/srmmu.c:300:9: error: variable 'pud' set but not used
> >> [-Werror=unused-but-set-variable]
> 300 |  pud_t *pud;
> 
> This warning is caused by misprint in the page table traversal in
> srmmu_nocache_init() function which accessed a PMD entry using PGD rather
> than PUD.
> Since sparc32 has only 3 page table levels, the PGD and PUD are essentially
> the same and usage of __nocache_fix() removed the type checking.
> 
> Use PUD for the consistency and to silence the compiler warning.
> 
> ...
>
> --- a/arch/sparc/mm/srmmu.c
> +++ b/arch/sparc/mm/srmmu.c
> @@ -304,7 +304,7 @@ static void __init srmmu_nocache_init(void)
>  		pgd = pgd_offset_k(vaddr);
>  		p4d = p4d_offset(__nocache_fix(pgd), vaddr);
>  		pud = pud_offset(__nocache_fix(p4d), vaddr);
> -		pmd = pmd_offset(__nocache_fix(pgd), vaddr);
> +		pmd = pmd_offset(__nocache_fix(pud), vaddr);
>  		pte = pte_offset_kernel(__nocache_fix(pmd), vaddr);
>  
>  		pteval = ((paddr >> 4) | SRMMU_ET_PTE | SRMMU_PRIV);

I added

Fixes: 7235db268a2777bc38 ("sparc32: use pgtable-nopud instead of 4level-fixup")

and, after a bit of thought,

Cc: <stable@vger.kernel.org>

Because that's a fairly scary-looking warning.