[PATCH 2/3] nodemask.h: Fix compilation error with GCC12

[Christophe de Dinechin <dinechin@redhat.com>]

With gcc version 12.0.1 20220401 (Red Hat 12.0.1-0), building with
defconfig results in the following compilation error:

|   CC      mm/swapfile.o
| mm/swapfile.c: In function ‘setup_swap_info’:
| mm/swapfile.c:2291:47: error: array subscript -1 is below array bounds
|  of ‘struct plist_node[]’ [-Werror=array-bounds]
|  2291 |                                 p->avail_lists[i].prio = 1;
|       |                                 ~~~~~~~~~~~~~~^~~
| In file included from mm/swapfile.c:16:
| ./include/linux/swap.h:292:27: note: while referencing ‘avail_lists’
|   292 |         struct plist_node avail_lists[]; /*
|       |                           ^~~~~~~~~~~

This is due to the compiler detecting that the mask in
node_states[__state] could theoretically be zero, which would lead to
first_node() returning -1 through find_first_bit.

I believe that the warning/error is legitimate. I first tried
adding a test to check that the node mask is not emtpy, since a
similar test exists in the case where MAX_NUMNODES == 1.

However, adding the if statement causes other warnings to appear in
for_each_cpu_node_but, because it introduces a dangling else
ambiguity. And unfortunately, GCC is not smart enough to detect that the
added test makes the case where (node) == -1 impossible, so it still
complains with the same message.

This is why I settled on replacing that with a harmless, but relatively
useless (node) >= 0 test. Based on the warning for the dangling else, I
also decided to fix the case where MAX_NUMNODES == 1 by moving the
condition inside the for loop. It will still only be tested once. This
ensures that the meaning of an else following for_each_node_mask
or derivatives would not silently have a different meaning depending on
the configuration.

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
---
 include/linux/nodemask.h | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
index 567c3ddba2c4..c6199dbe2591 100644
--- a/include/linux/nodemask.h
+++ b/include/linux/nodemask.h
@@ -375,14 +375,13 @@ static inline void __nodes_fold(nodemask_t *dstp, const nodemask_t *origp,
 }
 
 #if MAX_NUMNODES > 1
-#define for_each_node_mask(node, mask)			\
-	for ((node) = first_node(mask);			\
-		(node) < MAX_NUMNODES;			\
-		(node) = next_node((node), (mask)))
+#define for_each_node_mask(node, mask)				    \
+	for ((node) = first_node(mask);				    \
+	     (node >= 0) && (node) < MAX_NUMNODES;		    \
+	     (node) = next_node((node), (mask)))
 #else /* MAX_NUMNODES == 1 */
-#define for_each_node_mask(node, mask)			\
-	if (!nodes_empty(mask))				\
-		for ((node) = 0; (node) < 1; (node)++)
+#define for_each_node_mask(node, mask)                                  \
+	for ((node) = 0; (node) < 1 && !nodes_empty(mask); (node)++)
 #endif /* MAX_NUMNODES */
 
 /*
-- 
2.35.1