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From: "Theodore Y. Ts'o" <>
To: Andrew Morton <>
Cc: Linux Filesystem Development List <>,
Subject: Re: [PATCH -v2] memcg: fix a crash in wb_workfn when a device disappears
Date: Tue, 7 Jan 2020 21:12:38 -0500
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Jan 07, 2020 at 03:33:48PM -0800, Andrew Morton wrote:
> On Fri, 27 Dec 2019 19:52:11 -0500 "Theodore Ts'o" <> wrote:
> > 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?

The most common way of triggering this will be hotremoval of a device
while writeback with memcg enabled is going on.  It was triggering
several times a day in a heavily loaded production environment.

> Is it worth a cc:stable?

Yes, I think so.

						- Ted

      reply index

Thread overview: 10+ 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 21:19   ` Theodore Y. Ts'o
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
2020-01-08  2:12     ` Theodore Y. Ts'o [this message]

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