Re: [patch 36/51] revert "mm: vmalloc use mutex for purge"

[Andrew Morton]

On Fri, 16 Jan 2009 10:43:12 +0100 Nick Piggin <npiggin@xxxxxxx> wrote:

> On Fri, Jan 16, 2009 at 10:38:56AM +0100, Christophe Saout wrote:
> > Hi Nick,
> > 
> > > Weird. It seem to be something to do with Xen (and btrfs? or was it reproduced
> > > without?).
> > 
> > I got this bug without btrfs.  Seen on both Xen x86_32 and x86_64.
> > 
> > Note that I also some a different issue with CONFIG_UNEVICTABLE_LRU.
> > Seems like Xen tears down current->mm early on process termination, so
> > that __get_user_pages in exit_mmap causes nasty messages when the
> > process had any mlocked pages.  (in fact, it somehow manages to get into
> > the swapping code and produces a null pointer dereference trying to get
> > a swap token)
> 
> There is an oops there, yes. I remember I patch we have, although it was
> specifically for kernel threads rather than this issue. Xen could easily
> have bigger issues if it is exiting the mm before that final get_user_pages.
> 
>  
> 
> > > Anyway, I agree with the revert for the moment, but I'm worried that it might
> > > be hiding another bug... I might add a few might_sleep and in_atomic warnings
> > > around the place to see if it might find the culprit without crashing machines.
> > 
> > If you need some testing, please tell me.  On a dual-core machine this
> > bug happens within few minutes of a compiler run.
> 
> Ok, thanks... I'll see if I can get to it next week.
> 
> ---
> 
> From: Dean Roe <roe@xxxxxxx>
> Subject: Prevent NULL pointer deref in grab_swap_token
> References: 159260
> 
> grab_swap_token() assumes that the current process has an mm struct,
> which is not true for kernel threads invoking get_user_pages().  Since
> this should be extremely rare, just return from grab_swap_token()
> without doing anything.
> 
> Signed-off-by: Dean Roe <roe@xxxxxxx>
> Acked-by: mason@xxxxxxx
> Acked-by: okir@xxxxxxx
> 
> 
>  mm/thrash.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/mm/thrash.c
> +++ b/mm/thrash.c
> @@ -31,6 +31,9 @@ void grab_swap_token(void)
>  	int current_interval;
>  
>  	global_faults++;
> +	if (current->mm == NULL)
> +		return;
> +
>  
>  	current_interval = global_faults - current->mm->faultstamp;
>  

Confused.  Why was there a random, seemingly-unrelated patch at the end
of this email?

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