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

[Nick Piggin <npiggin@suse.de>]

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@sgi.com>
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@sgi.com>
Acked-by: mason@suse.de
Acked-by: okir@suse.de


 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;