Mem Policy: add MPOL_F_MEMS_ALLOWED get_mempolicy() flag
Allow an application to query the memories allowed by its context.
Updated numa_memory_policy.txt to mention that applications can use this to
obtain allowed memories for constructing valid policies.
TODO: update out-of-tree libnuma wrapper[s], or maybe add a new
wrapper--e.g., numa_get_mems_allowed() ?
Also, update numa syscall man pages.
Tested with memtoy V>=0.13.
Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Acked-by: Christoph Lameter <clameter@sgi.com>
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 8242f52..dd49864 100644
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -302,31 +302,30 @@
Memory policies work within cpusets as described above. For memory policies
that require a node or set of nodes, the nodes are restricted to the set of
-nodes whose memories are allowed by the cpuset constraints. If the
-intersection of the set of nodes specified for the policy and the set of nodes
-allowed by the cpuset is the empty set, the policy is considered invalid and
-cannot be installed.
+nodes whose memories are allowed by the cpuset constraints. If the nodemask
+specified for the policy contains nodes that are not allowed by the cpuset, or
+the intersection of the set of nodes specified for the policy and the set of
+nodes with memory is the empty set, the policy is considered invalid
+and cannot be installed.
The interaction of memory policies and cpusets can be problematic for a
couple of reasons:
-1) the memory policy APIs take physical node id's as arguments. However, the
- memory policy APIs do not provide a way to determine what nodes are valid
- in the context where the application is running. An application MAY consult
- the cpuset file system [directly or via an out of tree, and not generally
- available, libcpuset API] to obtain this information, but then the
- application must be aware that it is running in a cpuset and use what are
- intended primarily as administrative APIs.
-
- However, as long as the policy specifies at least one node that is valid
- in the controlling cpuset, the policy can be used.
+1) the memory policy APIs take physical node id's as arguments. As mentioned
+ above, it is illegal to specify nodes that are not allowed in the cpuset.
+ The application must query the allowed nodes using the get_mempolicy()
+ API with the MPOL_F_MEMS_ALLOWED flag to determine the allowed nodes and
+ restrict itself to those nodes. However, the resources available to a
+ cpuset can be changed by the system administrator, or a workload manager
+ application, at any time. So, a task may still get errors attempting to
+ specify policy nodes, and must query the allowed memories again.
2) when tasks in two cpusets share access to a memory region, such as shared
memory segments created by shmget() of mmap() with the MAP_ANONYMOUS and
MAP_SHARED flags, and any of the tasks install shared policy on the region,
only nodes whose memories are allowed in both cpusets may be used in the
- policies. Again, obtaining this information requires "stepping outside"
- the memory policy APIs, as well as knowing in what cpusets other task might
- be attaching to the shared region, to use the cpuset information.
+ policies. Obtaining this information requires "stepping outside" the
+ memory policy APIs to use the cpuset information and requires that one
+ know in what cpusets other task might be attaching to the shared region.
Furthermore, if the cpusets' allowed memory sets are disjoint, "local"
allocation is the only valid policy.