mm/slab: factor out slab_args_unmergeable()

slab_mergeable() determines whether a slab cache can be merged, but it
should not be used when the cache is not fully created yet.

Extract the pre-cache-creation mergeability checks into
slab_args_unmergeable(), which evaluates kmem_cache_args, slab flags,
and slab_nomerge to determine if a cache will be mergeable before it is
created.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260127103151.21883-2-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: hygoni

mm/slab_common.c

@@ -171,24 +171,32 @@ int slab_unmergeable(struct kmem_cache *s)
 	return 0;
 }
 
-static struct kmem_cache *find_mergeable(unsigned int size, slab_flags_t flags,
-		const char *name, struct kmem_cache_args *args)
+static bool slab_args_unmergeable(struct kmem_cache_args *args,
+				  slab_flags_t flags)
 {
-	struct kmem_cache *s;
-	unsigned int align;
-
 	if (slab_nomerge)
-		return NULL;
+		return true;
 
 	if (args->ctor)
-		return NULL;
+		return true;
 
 	if (IS_ENABLED(CONFIG_HARDENED_USERCOPY) && args->usersize)
-		return NULL;
-
-	flags = kmem_cache_flags(flags, name);
+		return true;
 
 	if (flags & SLAB_NEVER_MERGE)
+		return true;
+
+	return false;
+}
+
+static struct kmem_cache *find_mergeable(unsigned int size, slab_flags_t flags,
+		const char *name, struct kmem_cache_args *args)
+{
+	struct kmem_cache *s;
+	unsigned int align;
+
+	flags = kmem_cache_flags(flags, name);
+	if (slab_args_unmergeable(args, flags))
 		return NULL;
 
 	size = ALIGN(size, sizeof(void *));