MPOL_PREFERRED policies that were created with an empty nodemask
(local allocation).
+MEMORY POLICY REFERENCE COUNTING
+
+To resolve use/free races, struct mempolicy contains an atomic reference
+count field. Internal interfaces, mpol_get()/mpol_put() increment and
+decrement this reference count, respectively. mpol_put() will only free
+the structure back to the mempolicy kmem cache when the reference count
+goes to zero.
+
+When a new memory policy is allocated, it's reference count is initialized
+to '1', representing the reference held by the task that is installing the
+new policy. When a pointer to a memory policy structure is stored in another
+structure, another reference is added, as the task's reference will be dropped
+on completion of the policy installation.
+
+During run-time "usage" of the policy, we attempt to minimize atomic operations
+on the reference count, as this can lead to cache lines bouncing between cpus
+and NUMA nodes. "Usage" here means one of the following:
+
+1) querying of the policy, either by the task itself [using the get_mempolicy()
+ API discussed below] or by another task using the /proc/<pid>/numa_maps
+ interface.
+
+2) examination of the policy to determine the policy mode and associated node
+ or node lists, if any, for page allocation. This is considered a "hot
+ path". Note that for MPOL_BIND, the "usage" extends across the entire
+ allocation process, which may sleep during page reclaimation, because the
+ BIND policy nodemask is used, by reference, to filter ineligible nodes.
+
+We can avoid taking an extra reference during the usages listed above as
+follows:
+
+1) we never need to get/free the system default policy as this is never
+ changed nor freed, once the system is up and running.
+
+2) for querying the policy, we do not need to take an extra reference on the
+ target task's task policy nor vma policies because we always acquire the
+ task's mm's mmap_sem for read during the query. The set_mempolicy() and
+ mbind() APIs [see below] always acquire the mmap_sem for write when
+ installing or replacing task or vma policies. Thus, there is no possibility
+ of a task or thread freeing a policy while another task or thread is
+ querying it.
+
+3) Page allocation usage of task or vma policy occurs in the fault path where
+ we hold them mmap_sem for read. Again, because replacing the task or vma
+ policy requires that the mmap_sem be held for write, the policy can't be
+ freed out from under us while we're using it for page allocation.
+
+4) Shared policies require special consideration. One task can replace a
+ shared memory policy while another task, with a distinct mmap_sem, is
+ querying or allocating a page based on the policy. To resolve this
+ potential race, the shared policy infrastructure adds an extra reference
+ to the shared policy during lookup while holding a spin lock on the shared
+ policy management structure. This requires that we drop this extra
+ reference when we're finished "using" the policy. We must drop the
+ extra reference on shared policies in the same query/allocation paths
+ used for non-shared policies. For this reason, shared policies are marked
+ as such, and the extra reference is dropped "conditionally"--i.e., only
+ for shared policies.
+
+ Because of this extra reference counting, and because we must lookup
+ shared policies in a tree structure under spinlock, shared policies are
+ more expensive to use in the page allocation path. This is expecially
+ true for shared policies on shared memory regions shared by tasks running
+ on different NUMA nodes. This extra overhead can be avoided by always
+ falling back to task or system default policy for shared memory regions,
+ or by prefaulting the entire shared memory region into memory and locking
+ it down. However, this might not be appropriate for all applications.
+
MEMORY POLICY APIs
Linux supports 3 system calls for controlling memory policy. These APIS
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff