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>

diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt
index 6719d64..13cca5a 100644 (file)
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -147,35 +147,18 @@ Components of Memory Policies
 
    Linux memory policy supports the following 4 behavioral modes:
 
-       Default Mode--MPOL_DEFAULT:  The behavior specified by this mode is
-       context or scope dependent.
-
-           As mentioned in the Policy Scope section above, during normal
-           system operation, the System Default Policy is hard coded to
-           contain the Default mode.
-
-           In this context, default mode means "local" allocation--that is
-           attempt to allocate the page from the node associated with the cpu
-           where the fault occurs.  If the "local" node has no memory, or the
-           node's memory can be exhausted [no free pages available], local
-           allocation will "fallback to"--attempt to allocate pages from--
-           "nearby" nodes, in order of increasing "distance".
-
-               Implementation detail -- subject to change:  "Fallback" uses
-               a per node list of sibling nodes--called zonelists--built at
-               boot time, or when nodes or memory are added or removed from
-               the system [memory hotplug].  These per node zonelist are
-               constructed with nodes in order of increasing distance based
-               on information provided by the platform firmware.
-
-           When a task/process policy or a shared policy contains the Default
-           mode, this also means "local allocation", as described above.
-
-           In the context of a VMA, Default mode means "fall back to task
-           policy"--which may or may not specify Default mode.  Thus, Default
-           mode can not be counted on to mean local allocation when used
-           on a non-shared region of the address space.  However, see
-           MPOL_PREFERRED below.
+       Default Mode--MPOL_DEFAULT:  This mode is only used in the memory
+       policy APIs.  Internally, MPOL_DEFAULT is converted to the NULL
+       memory policy in all policy scopes.  Any existing non-default policy
+       will simply be removed when MPOL_DEFAULT is specified.  As a result,
+       MPOL_DEFAULT means "fall back to the next most specific policy scope."
+
+           For example, a NULL or default task policy will fall back to the
+           system default policy.  A NULL or default vma policy will fall
+           back to the task policy.
+
+           When specified in one of the memory policy APIs, the Default mode
+           does not use the optional set of nodes.
 
            It is an error for the set of nodes specified for this policy to
            be non-empty.
@@ -187,19 +170,18 @@ Components of Memory Policies
 
        MPOL_PREFERRED:  This mode specifies that the allocation should be
        attempted from the single node specified in the policy.  If that
-       allocation fails, the kernel will search other nodes, exactly as
-       it would for a local allocation that started at the preferred node
-       in increasing distance from the preferred node.  "Local" allocation
-       policy can be viewed as a Preferred policy that starts at the node
+       allocation fails, the kernel will search other nodes, in order of
+       increasing distance from the preferred node based on information
+       provided by the platform firmware.
        containing the cpu where the allocation takes place.
 
            Internally, the Preferred policy uses a single node--the
            preferred_node member of struct mempolicy.  A "distinguished
            value of this preferred_node, currently '-1', is interpreted
            as "the node containing the cpu where the allocation takes
-           place"--local allocation.  This is the way to specify
-           local allocation for a specific range of addresses--i.e. for
-           VMA policies.
+           place"--local allocation.  "Local" allocation policy can be
+           viewed as a Preferred policy that starts at the node containing
+           the cpu where the allocation takes place.
 
            It is possible for the user to specify that local allocation is
            always preferred by passing an empty nodemask with this mode.

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a237295..fea4a5d 100644 (file)
--- 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);