mempolicy: use MPOL_PREFERRED for system-wide default policy

Currently, when one specifies MPOL_DEFAULT via a NUMA memory policy API
[set_mempolicy(), mbind() and internal versions], the kernel simply installs a
NULL struct mempolicy pointer in the appropriate context: task policy, vma
policy, or shared policy.  This causes any use of that policy to "fall back"
to the next most specific policy scope.

The only use of MPOL_DEFAULT to mean "local allocation" is in the system
default policy.  This requires extra checks/cases for MPOL_DEFAULT in many
mempolicy.c functions.

There is another, "preferred" way to specify local allocation via the APIs.
That is using the MPOL_PREFERRED policy mode with an empty nodemask.
Internally, the empty nodemask gets converted to a preferred_node id of '-1'.
All internal usage of MPOL_PREFERRED will convert the '-1' to the id of the
node local to the cpu where the allocation occurs.

System default policy, except during boot, is hard-coded to "local
allocation".  By using the MPOL_PREFERRED mode with a negative value of
preferred node for system default policy, MPOL_DEFAULT will never occur in the
'policy' member of a struct mempolicy.  Thus, we can remove all checks for
MPOL_DEFAULT when converting policy to a node id/zonelist in the allocation
paths.

In slab_node() return local node id when policy pointer is NULL.  No need to
set a pol value to take the switch default.  Replace switch default with
BUG()--i.e., shouldn't happen.

With this patch MPOL_DEFAULT is only used in the APIs, including internal
calls to do_set_mempolicy() and in the display of policy in
/proc/<pid>/numa_maps.  It always means "fall back" to the the next most
specific policy scope.  This simplifies the description of memory policies
quite a bit, with no visible change in behavior.

get_mempolicy() continues to return MPOL_DEFAULT and an empty nodemask when
the requested policy [task or vma/shared] is NULL.  These are the values one
would supply via set_mempolicy() or mbind() to achieve that condition--default
behavior.

This patch updates Documentation to reflect this change.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 42 insertions, 26 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a237295f8190..fea4a5da6e44 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -104,9 +104,13 @@ static struct kmem_cache *sn_cache;
    policied. */
 enum zone_type policy_zone = 0;
 
+/*
+ * run-time system-wide default policy => local allocation
+ */
 struct mempolicy default_policy = {
         .refcnt = ATOMIC_INIT(1), /* never free it */
-        .mode   = MPOL_DEFAULT,
+        .mode = MPOL_PREFERRED,
+        .v =  { .preferred_node =  -1 },
 };
 
 static const struct mempolicy_operations {
@@ -189,7 +193,7 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
         if (mode == MPOL_DEFAULT) {
                 if (nodes && !nodes_empty(*nodes))
                         return ERR_PTR(-EINVAL);
-                return NULL;
+                return NULL;    /* simply delete any existing policy */
         }
         VM_BUG_ON(!nodes);
 
@@ -246,7 +250,6 @@ void __mpol_put(struct mempolicy *p)
 {
         if (!atomic_dec_and_test(&p->refcnt))
                 return;
-        p->mode = MPOL_DEFAULT;
         kmem_cache_free(policy_cache, p);
 }
 
@@ -626,13 +629,16 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
         return 0;
 }
 
-/* Fill a zone bitmap for a policy */
-static void get_zonemask(struct mempolicy *p, nodemask_t *nodes)
+/*
+ * Return nodemask for policy for get_mempolicy() query
+ */
+static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes)
 {
         nodes_clear(*nodes);
+        if (p == &default_policy)
+                return;
+
         switch (p->mode) {
-        case MPOL_DEFAULT:
-                break;
         case MPOL_BIND:
                 /* Fall through */
         case MPOL_INTERLEAVE:
@@ -686,6 +692,11 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
         }
 
         if (flags & MPOL_F_ADDR) {
+                /*
+                 * Do NOT fall back to task policy if the
+                 * vma/shared policy at addr is NULL.  We
+                 * want to return MPOL_DEFAULT in this case.
+                 */
                 down_read(&mm->mmap_sem);
                 vma = find_vma_intersection(mm, addr, addr+1);
                 if (!vma) {
@@ -700,7 +711,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
                 return -EINVAL;
 
         if (!pol)
-                pol = &default_policy;
+                pol = &default_policy;  /* indicates default behavior */
 
         if (flags & MPOL_F_NODE) {
                 if (flags & MPOL_F_ADDR) {
@@ -715,8 +726,11 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
                         err = -EINVAL;
                         goto out;
                 }
-        } else
-                *policy = pol->mode | pol->flags;
+        } else {
+                *policy = pol == &default_policy ? MPOL_DEFAULT :
+                                                pol->mode;
+                *policy |= pol->flags;
+        }
 
         if (vma) {
                 up_read(&current->mm->mmap_sem);
@@ -725,7 +739,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
 
         err = 0;
         if (nmask)
-                get_zonemask(pol, nmask);
+                get_policy_nodemask(pol, nmask);
 
  out:
         mpol_cond_put(pol);
@@ -1286,8 +1300,7 @@ static struct mempolicy *get_vma_policy(struct task_struct *task,
                                                                         addr);
                         if (vpol)
                                 pol = vpol;
-                } else if (vma->vm_policy &&
-                                vma->vm_policy->mode != MPOL_DEFAULT)
+                } else if (vma->vm_policy)
                         pol = vma->vm_policy;
         }
         if (!pol)
@@ -1334,7 +1347,6 @@ static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy)
                         nd = first_node(policy->v.nodes);
                 break;
         case MPOL_INTERLEAVE: /* should not happen */
-        case MPOL_DEFAULT:
                 nd = numa_node_id();
                 break;
         default:
@@ -1369,9 +1381,15 @@ static unsigned interleave_nodes(struct mempolicy *policy)
  */
 unsigned slab_node(struct mempolicy *policy)
 {
-        unsigned short pol = policy ? policy->mode : MPOL_DEFAULT;
+        if (!policy)
+                return numa_node_id();
+
+        switch (policy->mode) {
+        case MPOL_PREFERRED:
+                if (unlikely(policy->v.preferred_node >= 0))
+                        return policy->v.preferred_node;
+                return numa_node_id();
 
-        switch (pol) {
         case MPOL_INTERLEAVE:
                 return interleave_nodes(policy);
 
@@ -1390,13 +1408,8 @@ unsigned slab_node(struct mempolicy *policy)
                 return zone->node;
         }
 
-        case MPOL_PREFERRED:
-                if (policy->v.preferred_node >= 0)
-                        return policy->v.preferred_node;
-                /* Fall through */
-
         default:
-                return numa_node_id();
+                BUG();
         }
 }
 
@@ -1650,8 +1663,6 @@ int __mpol_equal(struct mempolicy *a, struct mempolicy *b)
         if (a->mode != MPOL_DEFAULT && !mpol_match_intent(a, b))
                 return 0;
         switch (a->mode) {
-        case MPOL_DEFAULT:
-                return 1;
         case MPOL_BIND:
                 /* Fall through */
         case MPOL_INTERLEAVE:
@@ -1828,7 +1839,7 @@ void mpol_shared_policy_init(struct shared_policy *info, unsigned short policy,
         if (policy != MPOL_DEFAULT) {
                 struct mempolicy *newpol;
 
-                /* Falls back to MPOL_DEFAULT on any error */
+                /* Falls back to NULL policy [MPOL_DEFAULT] on any error */
                 newpol = mpol_new(policy, flags, policy_nodes);
                 if (!IS_ERR(newpol)) {
                         /* Create pseudo-vma that contains just the policy */
@@ -1952,9 +1963,14 @@ static inline int mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
         char *p = buffer;
         int l;
         nodemask_t nodes;
-        unsigned short mode = pol ? pol->mode : MPOL_DEFAULT;
+        unsigned short mode;
         unsigned short flags = pol ? pol->flags : 0;
 
+        if (!pol || pol == &default_policy)
+                mode = MPOL_DEFAULT;
+        else
+                mode = pol->mode;
+
         switch (mode) {
         case MPOL_DEFAULT:
                 nodes_clear(nodes);