linux-kernel - Re: running get_user_pages() from kernel thread

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Date:	Tue, 16 Jun 2009 19:38:39 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>, nickpiggin@...oo.com.au
Subject: Re: running get_user_pages() from kernel thread

On Tue, 16 Jun 2009, Andrea Arcangeli wrote:
> On Tue, Jun 16, 2009 at 09:05:27PM +0300, Izik Eidus wrote:
> > So the question is: is this thing is by desgin? (that kernel thread cant 
> > call get_user_pages???), should i use something like switch_mm()??
> 
> I think switch_mm trick should be used for page faults, but gup
> shouldn't require it because it gets the 'mm' as parameter and the
> current->mm has to be irrelevant. current->mm is only relevant for
> gup-fast (obviously :). So I think the only bit that needs fixing is
> grab_swap_token to not run if current->mm is null.

Looks like Izik and I hit the same problem (otherwise running well):
I too decided we needn't do more than avoid the issue in grab_swap_token.
(I've a feeling someone has hit this issue before with some other thread,
though I've no idea which - does RHEL include a patch like this perhaps?).

[PATCH] mm: grab_swap_token back out if no mm

If a kthread happens to use get_user_pages() on an mm (as KSM does),
there's a chance that it will end up trying to read in a swap page,
and oops in grab_swap_token() because the kthread has no mm:
nothing clever, just avoid that case.

Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
---

 mm/thrash.c |    3 +++
 1 file changed, 3 insertions(+)

--- 2.6.30-mm1/mm/thrash.c	2007-07-09 00:32:17.000000000 +0100
+++ linux/mm/thrash.c	2009-06-15 19:44:53.000000000 +0100
@@ -30,6 +30,9 @@ void grab_swap_token(void)
 {
 	int current_interval;
 
+	if (!current->mm)	/* kthread doing get_user_pages on an mm */
+		return;
+
 	global_faults++;
 
 	current_interval = global_faults - current->mm->faultstamp;
--
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