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

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Date: Mon, 29 Jan 2024 16:45:22 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.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 12:51:59 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> End result: what simple_lookup() does is say "oh, you didn't have the
> file, so it's by definition a negative dentry", and thus all it does
> is to do "d_add(dentry, NULL)".
> 
> Anyway, removing this was painful. I initially thought "I'll just
> remove the calls". But it all ended up cascading into "that's also
> wrong".
> 
> So now I have a patch that tries to fix this all up, and it looks like thisL:
> 
>  1 file changed, 50 insertions(+), 219 deletions(-)

Thanks, much appreciated.

> 
> because it basically removed all the old code, and replaced it with
> much simpler code.
> 
> I'm including the patch here as an attachment, but I want to note very
> clearly that this *builds* for me, and it looks a *lot* more obvious
> and correct than the old code did, but I haven't tested it. AT ALL.

I'm going to stare at them as I test them. Because I want to understand
them. I may come back with questions.

> 
> Also note that it depends on my previous patches, so I guess I'll
> include them here again just to make it unambiguous.
> 
> Finally - this does *not* fix up the refcounting. I still think the
> SRCU stuff is completely broken. But that's another headache. But at
> least now the *lookup* parts look like they DTRT wrt eventfs_mutex.
> 
> The SRCU logic from the directory iteration parts still needs crapectomy.

I think the not dropping the mutex lock lets me get rid of the SRCU. I
added the SRCU when I was hitting the deadlocks with the iput code which
I'm not hitting anymore. So getting rid of the SRCU shouldn't be hard.

> 
> AGAIN: these patches (ie particularly that last one - 0004) were all
> done entirely "blindly" - I've looked at the code, and fixed the bugs
> and problems I've seen by pure code inspection.
> 
> That's great, but it really means that it's all untested. It *looks*
> better than the old code, but there may be some silly gotcha that I
> have missed.

I'll let you know. 

Oh, does b4 handle attachments? Because this breaks the patchwork flow.
I haven't used b4 yet.

Thanks,

-- Steve