Re: [PATCH] dm bufio: avoid false-positive Wmaybe-uninitialized warning

[Nathan Chancellor <natechancellor@gmail.com>]

On Thu, Jun 07, 2018 at 10:03:44PM -0700, Nathan Chancellor wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> commit 590347e4000356f55eb10b03ced2686bd74dab40 upstream.
> 
> gcc-6.3 and earlier show a new warning after a seemingly unrelated
> change to the arm64 PAGE_KERNEL definition:
> 
> In file included from drivers/md/dm-bufio.c:14:0:
> drivers/md/dm-bufio.c: In function 'alloc_buffer':
> include/linux/sched/mm.h:182:56: warning: 'noio_flag' may be used uninitialized in this function [-Wmaybe-uninitialized]
>   current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags;
>                                                         ^
> 
> The same warning happened earlier on linux-3.18 for MIPS and I did a
> workaround for that, but now it's come back.
> 
> gcc-7 and newer are apparently smart enough to figure this out, and
> other architectures don't show it, so the best I could come up with is
> to rework the caller slightly in a way that makes it obvious enough to
> all arm64 compilers what is happening here.
> 
> Fixes: 41acec624087 ("arm64: kpti: Make use of nG dependent on arm64_kernel_unmapped_at_el0()")
> Link: https://patchwork.kernel.org/patch/9692829/
> Cc: stable@vger.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> [snitzer: moved declarations inside conditional, altered vmalloc return]
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> [nc: Backport to 4.9, adjust context for lack of 19809c2da28a]
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>  drivers/md/dm-bufio.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> index 3ec647e8b9c6..35fd57fdeba9 100644
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -373,9 +373,6 @@ static void __cache_size_refresh(void)
>  static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
>  			       enum data_mode *data_mode)
>  {
> -	unsigned noio_flag;
> -	void *ptr;
> -
>  	if (c->block_size <= DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT) {
>  		*data_mode = DATA_MODE_SLAB;
>  		return kmem_cache_alloc(DM_BUFIO_CACHE(c), gfp_mask);
> @@ -399,16 +396,16 @@ static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
>  	 * all allocations done by this process (including pagetables) are done
>  	 * as if GFP_NOIO was specified.
>  	 */
> +	if (gfp_mask & __GFP_NORETRY) {
> +		unsigned noio_flag = memalloc_noio_save();
> +		void *ptr = __vmalloc(c->block_size, gfp_mask | __GFP_HIGHMEM,
> +				      PAGE_KERNEL);
>  
> -	if (gfp_mask & __GFP_NORETRY)
> -		noio_flag = memalloc_noio_save();
> -
> -	ptr = __vmalloc(c->block_size, gfp_mask | __GFP_HIGHMEM, PAGE_KERNEL);
> -
> -	if (gfp_mask & __GFP_NORETRY)
>  		memalloc_noio_restore(noio_flag);
> +		return ptr;
> +	}
>  
> -	return ptr;
> +	return __vmalloc(c->block_size, gfp_mask | __GFP_HIGHMEM, PAGE_KERNEL);
>  }
>  
>  /*
> -- 
> 2.17.1
> 

Gah, sorry, forgot to add a prefix. This should only be applied to 4.9,
it's already present in 4.14 and it isn't needed for 4.4.

Cheers!
Nathan