linux-kernel - Re: [PATCH] powerpc: fix "sz" set but not used

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Date:   Fri, 01 Mar 2019 14:24:16 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Qian Cai <cai@....pw>, benh@...nel.crashing.org, paulus@...ba.org
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Qian Cai <cai@....pw>
Subject: Re: [PATCH] powerpc: fix "sz" set but not used

Qian Cai <cai@....pw> writes:
> arch/powerpc/mm/hugetlbpage-hash64.c: In function '__hash_page_huge':
> arch/powerpc/mm/hugetlbpage-hash64.c:29:28: warning: variable 'sz' set
> but not used [-Wunused-but-set-variable]

It's always nice to know *why* it's not used.

It's unlikely, but not impossible, that it's a bug that the variable is
not used.

To find out why:

  $ git log --oneline -S sz arch/powerpc/mm/hugetlbpage-hash64.c
  0895ecda7942 powerpc/mm: Bring hugepage PTE accessor functions back into sync with normal accessors
  883a3e523672 powerpc/mm: Split hash MMU specific hugepage code into a new file

Then we look at those, and see the the first one did:

@@ -89,8 +62,7 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
        if (!cpu_has_feature(CPU_FTR_COHERENT_ICACHE))
                /* No CPU has hugepages but lacks no execute, so we
                 * don't need to worry about that case */
-               rflags = hash_huge_page_do_lazy_icache(rflags, __pte(old_pte),
-                                                      trap, sz);
+               rflags = hash_page_do_lazy_icache(rflags, __pte(old_pte), trap);


And if you read the commit log it's pretty clear the change was
deliberate and so sz should indeed be removed.

I've applied this as-is, no need to send a v2.

cheers

> diff --git a/arch/powerpc/mm/hugetlbpage-hash64.c b/arch/powerpc/mm/hugetlbpage-hash64.c
> index 2e6a8f9345d3..f6b09edc5e6e 100644
> --- a/arch/powerpc/mm/hugetlbpage-hash64.c
> +++ b/arch/powerpc/mm/hugetlbpage-hash64.c
> @@ -26,7 +26,7 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
>  	real_pte_t rpte;
>  	unsigned long vpn;
>  	unsigned long old_pte, new_pte;
> -	unsigned long rflags, pa, sz;
> +	unsigned long rflags, pa;
>  	long slot, offset;
>  
>  	BUG_ON(shift != mmu_psize_defs[mmu_psize].shift);
> @@ -73,7 +73,6 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
>  		offset = PTRS_PER_PMD;
>  	rpte = __real_pte(__pte(old_pte), ptep, offset);
>  
> -	sz = ((1UL) << shift);
>  	if (!cpu_has_feature(CPU_FTR_COHERENT_ICACHE))
>  		/* No CPU has hugepages but lacks no execute, so we
>  		 * don't need to worry about that case */
> -- 
> 2.17.2 (Apple Git-113)