From: Andrew Morton <firstname.lastname@example.org> To: "Theodore Ts'o" <email@example.com> Cc: Linux Filesystem Development List <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH -v2] memcg: fix a crash in wb_workfn when a device disappears Date: Tue, 7 Jan 2020 15:33:48 -0800 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On Fri, 27 Dec 2019 19:52:11 -0500 "Theodore Ts'o" <firstname.lastname@example.org> 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. > Is hotremoval of a device while tracing writeback the only known way of triggering this? Is it worth a cc:stable?
next prev parent reply other threads:[~2020-01-07 23:33 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-12-27 19:48 [PATCH] " Theodore Ts'o 2019-12-27 20:31 ` Theodore Ts'o 2019-12-27 21:16 ` kbuild test robot 2019-12-27 21:16 ` kbuild test robot 2019-12-27 21:19 ` Theodore Y. Ts'o 2019-12-27 21:19 ` Theodore Y. Ts'o 2019-12-27 22:32 ` kbuild test robot 2019-12-27 22:32 ` kbuild test robot 2019-12-28 0:52 ` [PATCH -v2] " Theodore Ts'o 2020-01-03 17:15 ` Theodore Y. Ts'o 2020-01-03 17:46 ` Chris Mason 2020-01-07 23:33 ` Andrew Morton [this message] 2020-01-08 2:12 ` Theodore Y. Ts'o
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