linux-kernel - Re: [bug ?] do_get

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Date:	Thu, 3 Jul 2008 14:44:33 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	John Blackwood <john.blackwood@...r.com>
cc:	linux-kernel@...r.kernel.org,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Joe Korty <joe.korty@...r.com>
Subject: Re: [bug ?] do_get_mempolicy()

On Thu, 3 Jul 2008, John Blackwood wrote:

> Hi Lee,
> 
> I'm having unexpected results with get_mempolicy(2) in 2.6.26, and
> I am hoping that you can either agree with me, or maybe comment on my
> misconceptions.
> 
> When I have a task with no special task mempolicy (the default mempolicy),
> when I call get_mempolicy(2), it returns a policy value of 2 (MPOL_BIND)
> with a NULL nodemask.
> 
> I believe that this is because of the code in do_get_mempolicy() that does:
> 
>  *policy |= pol->flags;
> 
> in the else case when flags do not contain MPOL_F_NODE.
> 
> I guess I don't understand why we are ORing in the pol->flags into the
> *policy value.  For example, when this is for the default_policy, the
> MPOL_F_LOCAL flag (which has a value of 2) gets stuffed into the *policy
> location, and a get_mempolicy(2) caller sees this as the MPOL_BIND
> mempolicy.
> 
> Maybe the "*policy |= pol->flags;" line should be removed ?
> 

You're right, the flags member of struct mempolicy has subsequently 
changed to carry "internal" flags that are not supposed to be exposed to 
userspace via the get_mempolicy() API.

The following patch probably fixes it.

Lee?

Signed-off-by: David Rientjes <rientjes@...gle.com>
---
 mm/mempolicy.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -729,7 +729,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
 	} else {
 		*policy = pol == &default_policy ? MPOL_DEFAULT :
 						pol->mode;
-		*policy |= pol->flags;
+		*policy |= (pol->flags & MPOL_MODE_FLAGS);
 	}
 
 	if (vma) {
--
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