slab: constify slab debug strings

Since the string passed to slab_debug is never modified, use pointers to
const char in all places where it is processed.

No functional changes intended.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Reviewed-by: Christoph Lameter <cl@gentwo.org>
Link: https://patch.msgid.link/819095b921f6ae03bb54fd69ee4020e2a3aef675.1761324765.git.ptesarik@suse.com
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: ptesarik

mm/slub.c

@@ -978,7 +978,7 @@ static slab_flags_t slub_debug = DEBUG_DEFAULT_FLAGS;
 static slab_flags_t slub_debug;
 #endif
 
-static char *slub_debug_string;
+static const char *slub_debug_string __ro_after_init;
 static int disable_higher_order_debug;
 
 /*
@@ -1785,8 +1785,8 @@ static inline int free_consistency_checks(struct kmem_cache *s,
  *
  * returns the start of next block if there's any, or NULL
  */
-static char *
-parse_slub_debug_flags(char *str, slab_flags_t *flags, char **slabs, bool init)
+static const char *
+parse_slub_debug_flags(const char *str, slab_flags_t *flags, const char **slabs, bool init)
 {
 	bool higher_order_disable = false;
 
@@ -1863,14 +1863,15 @@ parse_slub_debug_flags(char *str, slab_flags_t *flags, char **slabs, bool init)
 		return NULL;
 }
 
-static int __init setup_slub_debug(char *str)
+static int __init setup_slub_debug(char *val)
 {
 	slab_flags_t flags;
 	slab_flags_t global_flags;
-	char *saved_str;
-	char *slab_list;
+	const char *saved_str;
+	const char *slab_list;
 	bool global_slub_debug_changed = false;
 	bool slab_list_specified = false;
+	const char *str = val;
 
 	global_flags = DEBUG_DEFAULT_FLAGS;
 	if (*str++ != '=' || !*str)
@@ -1935,9 +1936,9 @@ __setup_param("slub_debug", slub_debug, setup_slub_debug, 0);
  */
 slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name)
 {
-	char *iter;
+	const char *iter;
 	size_t len;
-	char *next_block;
+	const char *next_block;
 	slab_flags_t block_flags;
 	slab_flags_t slub_debug_local = slub_debug;
 
@@ -1961,7 +1962,7 @@ slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name)
 			continue;
 		/* Found a block that has a slab list, search it */
 		while (*iter) {
-			char *end, *glob;
+			const char *end, *glob;
 			size_t cmplen;
 
 			end = strchrnul(iter, ',');