reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/colyli/linux-bcache.git/commit/?id=f7093093e7a41cc15a90a97581d77d4c4af54cc5
        git remote add colyli-bcache https://git.kernel.org/pub/scm/linux/kernel/git/colyli/linux-bcache.git
        git fetch --no-tags colyli-bcache for-next
        git checkout f7093093e7a41cc15a90a97581d77d4c4af54cc5
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash drivers/md/