linux-kernel - [PATCH 5.18 831/879] nodemask.h: fix compilation error with GCC12

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Message-Id: <20220607165026.976043805@linuxfoundation.org>
Date:   Tue,  7 Jun 2022 19:05:49 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christophe de Dinechin <christophe@...echin.org>,
        Christophe de Dinechin <dinechin@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Segall <bsegall@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 5.18 831/879] nodemask.h: fix compilation error with GCC12

From: Christophe de Dinechin <dinechin@...hat.com>

commit 37462a920392cb86541650a6f4121155f11f1199 upstream.

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.

Link: https://lkml.kernel.org/r/20220414150855.2407137-3-dinechin@redhat.com
Signed-off-by: Christophe de Dinechin <christophe@...echin.org>
Signed-off-by: Christophe de Dinechin <dinechin@...hat.com>
Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: Zhen Lei <thunder.leizhen@...wei.com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/nodemask.h |   13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

--- a/include/linux/nodemask.h
+++ b/include/linux/nodemask.h
@@ -375,14 +375,13 @@ static inline void __nodes_fold(nodemask
 }
 
 #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 */
 
 /*