linux-kernel - Re: [patch for-3.14] mm, mempolicy: fix mempolicy printing in numa

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Date:	Mon, 27 Jan 2014 11:03:30 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch for-3.14] mm, mempolicy: fix mempolicy printing in
 numa_maps

On Sat, Jan 25, 2014 at 07:12:35PM -0800, David Rientjes wrote:
> As a result of commit 5606e3877ad8 ("mm: numa: Migrate on reference 
> policy"), /proc/<pid>/numa_maps prints the mempolicy for any <pid> as 
> "prefer:N" for the local node, N, of the process reading the file.
> 
> This should only be printed when the mempolicy of <pid> is MPOL_PREFERRED 
> for node N.
> 
> If the process is actually only using the default mempolicy for local node 
> allocation, make sure "default" is printed as expected.
> 
> Reported-by: Robert Lippert <rlippert@...gle.com>
> Signed-off-by: David Rientjes <rientjes@...gle.com>

Hmm, it is using a preferred policy but I see your point as expectations
of an application parsing numa_maps have been broken.  The patch makes
non-obvious assumptions about how and when MPOL_F_MORON gets set which
could change in the future and be missed. Use this instead? It might need
to be changed again if there is a need to control whether automatic numa
balancing can be enabled or disabled on a per-process basis.

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index c2ccec0..c1a2573 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -120,6 +120,14 @@ static struct mempolicy default_policy = {
 
 static struct mempolicy preferred_node_policy[MAX_NUMNODES];
 
+/* Returns true if the policy is the default policy */
+static bool mpol_is_default(struct mempolicy *pol)
+{
+	return !pol ||
+		pol == &default_policy ||
+		pol == &preferred_node_policy[numa_node_id()];
+}
+
 static struct mempolicy *get_task_policy(struct task_struct *p)
 {
 	struct mempolicy *pol = p->mempolicy;
@@ -2856,7 +2864,7 @@ void mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
 	unsigned short mode = MPOL_DEFAULT;
 	unsigned short flags = 0;
 
-	if (pol && pol != &default_policy) {
+	if (!mpol_is_default(pol)) {
 		mode = pol->mode;
 		flags = pol->flags;
 	}
--
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