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

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Date: Mon, 29 Jan 2024 14:35:37 -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 14:21, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> But crashes with just a:
>
>  # ls /sys/kernel/tracing/events
>
> [   66.423983] ------------[ cut here ]------------
> [   66.426447] kernel BUG at fs/dcache.c:1876!

Duh.

That's a bit too much copy-and-paste by me.

So what is going on is that a ->lookup() function should *not* call
d_instantiate() at all, and the only reason it actually used to work
here was due to the incorrect "simple_lookup()", which basically did
all the preliminaries.

A ->lookup() should do 'd_add()' on the dentry.

So just replace all the d_instantiate() calls there with "d_add()"
instead. I think that will fix it.

Basically the "simple_lookup()" had done the "d_add(dentry, NULL)",
and at that point the "d_instantiate()" just exposed the inode and
turned the negative dentry into a positive one.

So "d_add()" is "I'm adding the inode to a new dentry under lookup".
And "d_instantiate()" is "I'm adding this inode to an existing dentry
that used to be negative"

And so the old "d_add(NULL)+d_instantiate(inode)" _kind_ of worked,
except it made that negative dentry visible for a short while.

And when I did the cleanup, I didn't think of this thing, so I left
the d_instantiate() calls as such, even though they now really need to
be d_add().

Hope that explains it.

And I hope there aren't any other stupid things I missed like that.

         Linus