I've run across a regression from v4.11. If I boot a v4.12-rc1 or later
kernel, make a large brd device and try to format it, it quickly slows
down to a crawl and then the OOM killer kicks in.

I ran a bisect and it landed here:

commit f09a06a193d942a12c1a33c153388b3962222006 (HEAD, refs/bisect/bad)
Author: Christoph Hellwig <hch@lst.de>
Date:   Wed Apr 5 19:21:16 2017 +0200

    brd: remove discard support
    
    It's just a in-driver reimplementation of writing zeroes to the pages,
    which fails if the discards aren't page aligned.
    
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Reviewed-by: Hannes Reinecke <hare@suse.com>
    Signed-off-by: Jens Axboe <axboe@fb.com>


I've been reproducing it in a VM with ~8G allocated to it:

I have a modprobe.d file with this in it:

    options brd rd_nr=1 rd_size=1073741824

I then just:

    # modprobe brd
    # mkfs -t ext2 /dev/ram0

It keels over pretty quickly after that.

My .config is attached.

Cheers,
-- 
Jeff Layton <jlayton@redhat.com>