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, 18 insertions, 36 deletions

diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt
index 6719d642653f..13cca5a3cf17 100644
--- 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.