linux-kernel - Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for

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Date: Mon, 29 Jan 2024 14:19:56 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev, lkp@...el.com, 
	linux-kernel@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Christian Brauner <brauner@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Ajay Kaher <ajay.kaher@...adcom.com>, linux-trace-kernel@...r.kernel.org
Subject: Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for_address

On Mon, 29 Jan 2024 at 13:45, Steven Rostedt <rostedt@...dmis.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