[RFC] cpuset: Enable changing of top_cpuset's mems_allowed nodemask

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

At present, top_cpuset.mems_allowed is same as node_states[N_MEMORY] and it
cannot be changed at the runtime. Maximum possible node_states[N_MEMORY]
also gets reflected in top_cpuset.effective_mems interface. It prevents some
one from removing or restricting memory placement which will be applicable
system wide on a given memory node through cpuset mechanism which might be
limiting. This solves the problem by enabling update_nodemask() function to
accept changes to top_cpuset.mems_allowed as well. Once changed, it also
updates the value of top_cpuset.effective_mems. Updates all it's task's
mems_allowed nodemask as well. It calls cpuset_inc() to make sure cpuset
is accounted for in the buddy allocator through cpusets_enabled() check.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
---
Tested for

* Enforcement of changed top_cpuset.mems_allowed
* Global mems_allowed cannot be changed till there are other
  cpusets present underneath the top root cpuset. I guess it
  is expected.

 kernel/cpuset.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index b308888..e8c105a 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1210,15 +1210,6 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
 	int retval;
 
 	/*
-	 * top_cpuset.mems_allowed tracks node_stats[N_MEMORY];
-	 * it's read-only
-	 */
-	if (cs == &top_cpuset) {
-		retval = -EACCES;
-		goto done;
-	}
-
-	/*
 	 * An empty mems_allowed is ok iff there are no tasks in the cpuset.
 	 * Since nodelist_parse() fails on an empty mask, we special case
 	 * that parsing.  The validate_change() call ensures that cpusets
@@ -1232,7 +1223,7 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
 			goto done;
 
 		if (!nodes_subset(trialcs->mems_allowed,
-				  top_cpuset.mems_allowed)) {
+				  node_states[N_MEMORY])) {
 			retval = -EINVAL;
 			goto done;
 		}
@@ -1250,6 +1241,16 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
 	cs->mems_allowed = trialcs->mems_allowed;
 	spin_unlock_irq(&callback_lock);
 
+	if (cs == &top_cpuset) {
+		spin_lock_irq(&callback_lock);
+		cs->effective_mems = trialcs->mems_allowed;
+		spin_unlock_irq(&callback_lock);
+
+		update_tasks_nodemask(cs);
+		cpuset_inc();
+		goto done;
+	}
+
 	/* use trialcs->mems_allowed as a temp variable */
 	update_nodemasks_hier(cs, &trialcs->mems_allowed);
 done:
-- 
1.8.3.1

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