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

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Date:	Tue, 16 Jun 2009 14:55:14 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	nickpiggin@...oo.com.au
Subject: Re: running get_user_pages() from kernel thread

Hugh Dickins wrote:
> 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?).

It looks very familiar, indeed.

RHEL 4 and 5 both have code like this in the swap token
code. I seem to remember submitting such code upstream,
too.

I have no idea why it never got upstream, maybe I was
drowned in Xen work at the time, or maybe the bug simply
didn't happen upstream for whatever reason.

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

Acked-by: Rik van Riel <riel@...hat.com>

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