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[PATCH 4.9 29/31] dm bufio: avoid false-positive Wmaybe-uninitialized warning

4.9-stable review patch.  If anyone has any objections, please let me know.


From: Arnd Bergmann <arnd@xxxxxxxx>

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@xxxxxxxxxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
[snitzer: moved declarations inside conditional, altered vmalloc return]
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
[nc: Backport to 4.9, adjust context for lack of 19809c2da28a]
Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

 drivers/md/dm-bufio.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

--- 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
 	 * 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)
+		return ptr;
+	}
-	return ptr;
+	return __vmalloc(c->block_size, gfp_mask | __GFP_HIGHMEM, PAGE_KERNEL);