slub: fix sysfs duplicate filename creation when slub_debug=O

When slub_debug=O is set.  It is possible to clear debug flags for an
"unmergeable" slab cache in kmem_cache_open().  It makes the "unmergeable"
cache became "mergeable" in sysfs_slab_add().

These caches will generate their "unique IDs" by create_unique_id(), but
it is possible to create identical unique IDs.  In my experiment,
sgpool-128, names_cache, biovec-256 generate the same ID ":Ft-0004096" and
the kernel reports "sysfs: cannot create duplicate filename
'/kernel/slab/:Ft-0004096'".

To repeat my experiment, set disable_higher_order_debug=1,
CONFIG_SLUB_DEBUG_ON=y in kernel-4.14.

Fix this issue by setting unmergeable=1 if slub_debug=O and the the
default slub_debug contains any no-merge flags.

call path:
kmem_cache_create()
  __kmem_cache_alias()	-> we set SLAB_NEVER_MERGE flags here
  create_cache()
    __kmem_cache_create()
      kmem_cache_open()	-> clear DEBUG_METADATA_FLAGS
      sysfs_slab_add()	-> the slab cache is mergeable now

  sysfs: cannot create duplicate filename '/kernel/slab/:Ft-0004096'
  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x60/0x7c
  Modules linked in:
  CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W       4.14.0-rc7ajb-00131-gd4c2e9f-dirty #123
  Hardware name: linux,dummy-virt (DT)
  task: ffffffc07d4e0080 task.stack: ffffff8008008000
  PC is at sysfs_warn_dup+0x60/0x7c
  LR is at sysfs_warn_dup+0x60/0x7c
  pc :  lr :  pstate: 60000145
  Call trace:
   sysfs_warn_dup+0x60/0x7c
   sysfs_create_dir_ns+0x98/0xa0
   kobject_add_internal+0xa0/0x294
   kobject_init_and_add+0x90/0xb4
   sysfs_slab_add+0x90/0x200
   __kmem_cache_create+0x26c/0x438
   kmem_cache_create+0x164/0x1f4
   sg_pool_init+0x60/0x100
   do_one_initcall+0x38/0x12c
   kernel_init_freeable+0x138/0x1d4
   kernel_init+0x10/0xfc
   ret_from_fork+0x10/0x18

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Miles Chen <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: milesdotchen

mm/slub.c

@@ -5706,6 +5706,10 @@ static int sysfs_slab_add(struct kmem_cache *s)
 		return 0;
 	}
 
+	if (!unmergeable && disable_higher_order_debug &&
+			(slub_debug & DEBUG_METADATA_FLAGS))
+		unmergeable = 1;
+
 	if (unmergeable) {
 		/*
 		 * Slabcache can never be merged so we can use the name proper.