On Fri, Dec 27, 2019 at 07:52:11PM -0500, Theodore Ts'o wrote: > Without memcg, there is a one-to-one mapping between the bdi and > bdi_writeback structures. In this world, things are fairly > straightforward; the first thing bdi_unregister() does is to shutdown > the bdi_writeback structure (or wb), and part of that writeback > ensures that no other work queued against the wb, and that the wb is > fully drained. > > With memcg, however, there is a one-to-many relationship between the > bdi and bdi_writeback structures; that is, there are multiple wb > objects which can all point to a single bdi. There is a refcount > which prevents the bdi object from being released (and hence, > unregistered). So in theory, the bdi_unregister() *should* only get > called once its refcount goes to zero (bdi_put will drop the refcount, > and when it is zero, release_bdi gets called, which calls > bdi_unregister). > > Unfortunately, del_gendisk() in block/gen_hd.c never got the memo > about the Brave New memcg World, and calls bdi_unregister directly. > It does this without informing the file system, or the memcg code, or > anything else. This causes the root wb associated with the bdi to be > unregistered, but none of the memcg-specific wb's are shutdown. So when > one of these wb's are woken up to do delayed work, they try to > dereference their wb->bdi->dev to fetch the device name, but > unfortunately bdi->dev is now NULL, thanks to the bdi_unregister() > called by del_gendisk(). As a result, *boom*. > > Fortunately, it looks like the rest of the writeback path is perfectly > happy with bdi->dev and bdi->owner being NULL, so the simplest fix is > to create a bdi_dev_name() function which can handle bdi->dev being > NULL. This also allows us to bulletproof the writeback tracepoints to > prevent them from dereferencing a NULL pointer and crashing the kernel > if one is tracing with memcg's enabled, and an iSCSI device dies or a > USB storage stick is pulled. > > Previous-Version-Link: https://lore.kernel.org/r/20191227194829.150110-1-tytso@xxxxxxx > Google-Bug-Id: 145475544 > Tested: fs smoke test > Signed-off-by: Theodore Ts'o <tytso@xxxxxxx> > --- > > Notes: > v2: add #include for linux/device.h > > fs/fs-writeback.c | 2 +- > include/linux/backing-dev.h | 10 +++++++++ > include/trace/events/writeback.h | 37 +++++++++++++++----------------- > mm/backing-dev.c | 1 + > 4 files changed, 29 insertions(+), 21 deletions(-) Ping? Any comments? Any objections if I carry this patch[1] in the ext4 tree? Or would it be better for Andrew to carry it in the linux-mm tree? [1] https://lore.kernel.org/k/20191227203117.152399-1-tytso@xxxxxxx - Ted
