block: implement CONFIG_DEBUG_BLOCK_EXT_DEVT

Extended devt introduces non-contiguos device numbers.  This patch
implements a debug option which forces most devt allocations to be
from the extended area and spreads them out.  This is enabled by
default if DEBUG_KERNEL is set and achieves...

1. Detects code paths in kernel or userland which expect predetermined
   consecutive device numbers.

2. When something goes wrong, avoid corruption as adding to the minor
   of earlier partition won't lead to the wrong but valid device.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0b504814e378..5a536f703a83 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -624,6 +624,22 @@ config BACKTRACE_SELF_TEST
 
           Say N if you are unsure.
 
+config DEBUG_BLOCK_EXT_DEVT
+        bool "Force extended block device numbers and spread them"
+        depends on DEBUG_KERNEL
+        depends on BLOCK
+        default y
+        help
+          Conventionally, block device numbers are allocated from
+          predetermined contiguous area.  However, extended block area
+          may introduce non-contiguous block device numbers.  This
+          option forces most block device numbers to be allocated from
+          the extended space and spreads them to discover kernel or
+          userland code paths which assume predetermined contiguous
+          device number allocation.
+
+          Say N if you are unsure.
+
 config LKDTM
         tristate "Linux Kernel Dump Test Tool Module"
         depends on DEBUG_KERNEL