Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for_address

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: kernel test robot <oliver.sang@intel.com>,
	oe-lkp@lists.linux.dev, lkp@intel.com,
	 linux-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	 Christian Brauner <brauner@kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	 Ajay Kaher <ajay.kaher@broadcom.com>,
	linux-trace-kernel@vger.kernel.org
Subject: Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for_address
Date: Mon, 29 Jan 2024 14:19:56 -0800	[thread overview]
Message-ID: <CAHk-=wgcy_8snS3znhTB2M_=rDg5gy8bQunKvhLse=AVPp6rOg@mail.gmail.com> (raw)
In-Reply-To: <20240129164522.72482faa@gandalf.local.home>

On Mon, 29 Jan 2024 at 13:45, Steven Rostedt <rostedt@goodmis.org> wrote:
> >  1 file changed, 50 insertions(+), 219 deletions(-)
>
> Thanks, much appreciated.

Well, I decided I might as well give it a test-run, and there's an
immediate deadlock on eventfs_mutex, because I missed removing it from
eventfs_find_events() when the callers now already hold it.

So at a minimum, it will require this patch on top:

  --- a/fs/tracefs/event_inode.c
  +++ b/fs/tracefs/event_inode.c
  @@ -230,7 +230,6 @@ static struct eventfs_inode
*eventfs_find_events(
   {
        struct eventfs_inode *ei;

  -     mutex_lock(&eventfs_mutex);
        do {
                // The parent is stable because we do not do renames
                dentry = dentry->d_parent;
  @@ -247,7 +246,6 @@
                }
                // Walk upwards until you find the events inode
        } while (!ei->is_events);
  -     mutex_unlock(&eventfs_mutex);

        update_top_events_attr(ei, dentry->d_sb);

to not deadlock immediately on the first lookup.

And honestly, there might be other such obvious "I missed that when
reading the code".

Let me reboot into a fixed system and do some more basic smoke-testing.

                Linus