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

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Message-ID: <CAHk-=wh97AkwaOkXoBgf0z8EP88ePffLnTcmmQXcY+AhFaFrnA@mail.gmail.com>
Date: Mon, 29 Jan 2024 19:56:52 -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 17:50, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So what I propose is that
>
>  - ei->dentry and ei->d_children[] need to die. Really. They are
> buggy. There is no way to save them. There never was.
>
>  - but we *can* introduce a new 'ei->events_dir' pointer that is
> *only* set by eventfs_create_events_dir(), and which is stable exactly
> because that function also does a dget() on it, so now the dentry will
> actually continue to exist reliably
>
> I think that works.

Well, it doesn't. I don't see where the bug is, but since Al is now
aware of the thread, maybe when he wakes up he will tell me where I've
gone wrong.

In the meantime I did do the pending tracefs pull, so the series has
changed a bit, and this is the rebased series on top of my current
public git tree.

It is still broken wrt 'events' directories. You don't even need the
"create, delete, create" sequence that Steven pointed out, just a
plain sequence of

 # cd /sys/kernel/tracing
 # ls events/kprobes/
 # echo 'p:sched schedule' >> kprobe_events

messes up - ie it's enough to just have 'lookup' create a negative
dentry by trying to look up 'events/kprobes/' before actually trying
to create that kprobe_events.

But I've been staring at this code for too long, so I'm posting this
just as a "it's broken, but _something_ like this", because I'm taking
a break from looking at this.

I'll get back to it tomorrow, but I hope that Al will show me the
error of my ways.

              Linus

View attachment "0005-eventfs-get-rid-of-dentry-pointers-without-refcounts.patch" of type "text/x-patch" (15652 bytes)

View attachment "0003-tracefs-dentry-lookup-crapectomy.patch" of type "text/x-patch" (12628 bytes)

View attachment "0001-tracefs-avoid-using-the-ei-dentry-pointer-unnecessar.patch" of type "text/x-patch" (3561 bytes)

View attachment "0002-eventfsfs-initialize-the-tracefs-inode-properly.patch" of type "text/x-patch" (2548 bytes)

View attachment "0004-eventfs-remove-unused-d_parent-pointer-field.patch" of type "text/x-patch" (1854 bytes)