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Subject: Re: [PATCH] powerpc/powernv: fix variable "c" set but not used
To:     Qian Cai <cai@lca.pw>, benh@kernel.crashing.org, paulus@samba.org,
        mpe@ellerman.id.au
Cc:     aik@ozlabs.ru, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
References: <20190523023141.2973-1-cai@lca.pw>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <d0512822-ca22-75ec-3dd9-1024001632f5@c-s.fr>
Date:   Thu, 23 May 2019 09:26:53 +0200
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Le 23/05/2019 à 04:31, Qian Cai a écrit :
> The commit 58629c0dc349 ("powerpc/powernv/npu: Fault user page into the
> hypervisor's pagetable") introduced a variable "c" to be used in
> __get_user() and __get_user_nocheck() which need to stay as macros for
> performance reasons, and "c" is not actually used in
> pnv_npu2_handle_fault(),
> 
> arch/powerpc/platforms/powernv/npu-dma.c: In function 'pnv_npu2_handle_fault':
> arch/powerpc/platforms/powernv/npu-dma.c:1122:7: warning: variable 'c'
> set but not used [-Wunused-but-set-variable]
> 
> Fixed it by appending the __maybe_unused attribute, so compilers would
> ignore it.

You are not fixing the problem, you are just hiding it.

If the result of __get_user() is unneeded, it means __get_user() is not 
the good function to use.

Should use fault_in_pages_readable() instead.

A similar warning was fixed in commit 9f9eae5ce717 ("powerpc/kvm: Prefer 
fault_in_pages_readable function")

See 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/powerpc?id=9f9eae5ce

> 
> Signed-off-by: Qian Cai <cai@lca.pw>

You should add a Fixes: tag

58629c0dc349 ("powerpc/powernv/npu: Fault user page into the 
hypervisor's pagetable")

Christophe

> ---
>   arch/powerpc/platforms/powernv/npu-dma.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
> index 495550432f3d..5bbe59573ee6 100644
> --- a/arch/powerpc/platforms/powernv/npu-dma.c
> +++ b/arch/powerpc/platforms/powernv/npu-dma.c
> @@ -1119,7 +1119,8 @@ int pnv_npu2_handle_fault(struct npu_context *context, uintptr_t *ea,
>   	int i, is_write;
>   	struct page *page[1];
>   	const char __user *u;
> -	char c;
> +	/* To silence a -Wunused-but-set-variable warning. */
> +	char c __maybe_unused;
>   
>   	/* mmap_sem should be held so the struct_mm must be present */
>   	struct mm_struct *mm = context->mm;
>