slab: Warn on duplicate cache names when DEBUG_VM=y

Duplicate slab cache names can create havoc for userspace tooling that
expects slab cache names to be unique [1]. This is a reasonable
expectation.

Sadly, too many duplicate name problems are out there in the wild, so
simply warn instead of pr_err() + failing the sanity check.

[ vbabka@suse.cz: change to WARN_ON(), see the discussion at [2] ]

Link: https://lore.kernel.org/linux-fsdevel/2d1d053da1cafb3e7940c4f25952da4f0af34e38.1722293276.git.osandov@fb.com/ [1]
Link: https://lore.kernel.org/all/20240807090746.2146479-1-pedro.falcato@gmail.com/ [2]
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slab_common.c

@@ -88,6 +88,19 @@ unsigned int kmem_cache_size(struct kmem_cache *s)
 EXPORT_SYMBOL(kmem_cache_size);
 
 #ifdef CONFIG_DEBUG_VM
+
+static bool kmem_cache_is_duplicate_name(const char *name)
+{
+	struct kmem_cache *s;
+
+	list_for_each_entry(s, &slab_caches, list) {
+		if (!strcmp(s->name, name))
+			return true;
+	}
+
+	return false;
+}
+
 static int kmem_cache_sanity_check(const char *name, unsigned int size)
 {
 	if (!name || in_interrupt() || size > KMALLOC_MAX_SIZE) {
@@ -95,6 +108,10 @@ static int kmem_cache_sanity_check(const char *name, unsigned int size)
 		return -EINVAL;
 	}
 
+	/* Duplicate names will confuse slabtop, et al */
+	WARN(kmem_cache_is_duplicate_name(name),
+			"kmem_cache of name '%s' already exists\n", name);
+
 	WARN_ON(strchr(name, ' '));	/* It confuses parsers */
 	return 0;
 }