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
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -147,35 +147,18 @@
Linux memory policy supports the following 4 behavioral modes:
- Default Mode--MPOL_DEFAULT: The behavior specified by this mode is
- context or scope dependent.
+ 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."
- As mentioned in the Policy Scope section above, during normal
- system operation, the System Default Policy is hard coded to
- contain the Default mode.
+ 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.
- 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.
+ 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 @@
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.