Re: [PATCH] powerpc/powernv: fix variable "c" set but not used

[Christophe Leroy]

Le 23/05/2019 Ã 04:31, Qian Cai a ÃcritÂ:
> 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@xxxxxx>

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;