linux-kernel - [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

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Date:   Tue,  1 Feb 2022 14:29:24 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Justin Forbes <jforbes@...hat.com>,
        Rafael Aquini <aquini@...hat.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

The gcc 12 compiler reports a warning on the following code:

    static inline struct mem_section *__nr_to_section(unsigned long nr)
    {
    #ifdef CONFIG_SPARSEMEM_EXTREME
        if (!mem_section)
                return NULL;
    #endif
       :

With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is

    extern struct mem_section **mem_section;

Obviously, mem_section cannot be NULL, but *mem_section can be if memory
hasn't been allocated for the dynamic mem_section[] array yet. Fix this
warning by checking for "!*mem_section" instead.

Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 include/linux/mmzone.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index aed44e9b5d89..bd1b19925f3b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
 static inline struct mem_section *__nr_to_section(unsigned long nr)
 {
 #ifdef CONFIG_SPARSEMEM_EXTREME
-	if (!mem_section)
+	if (!*mem_section)
 		return NULL;
 #endif
 	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
-- 
2.27.0