mempolicy: document {set|get}_policy() vm_ops APIs

Document mempolicy return value reference semantics assumed by the rest of the
mempolicy code for the set_ and get_policy vm_ops in <linux/mm.h>--where the
prototypes are defined--to inform any future mempolicy vm_op writers what the
rest of the subsystem expects of them.

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, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bc0ad24cf8c0..8b7f4a5d4f6a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -172,7 +172,25 @@ struct vm_operations_struct {
          * writable, if an error is returned it will cause a SIGBUS */
         int (*page_mkwrite)(struct vm_area_struct *vma, struct page *page);
 #ifdef CONFIG_NUMA
+        /*
+         * set_policy() op must add a reference to any non-NULL @new mempolicy
+         * to hold the policy upon return.  Caller should pass NULL @new to
+         * remove a policy and fall back to surrounding context--i.e. do not
+         * install a MPOL_DEFAULT policy, nor the task or system default
+         * mempolicy.
+         */
         int (*set_policy)(struct vm_area_struct *vma, struct mempolicy *new);
+
+        /*
+         * get_policy() op must add reference [mpol_get()] to any policy at
+         * (vma,addr) marked as MPOL_SHARED.  The shared policy infrastructure
+         * in mm/mempolicy.c will do this automatically.
+         * get_policy() must NOT add a ref if the policy at (vma,addr) is not
+         * marked as MPOL_SHARED. vma policies are protected by the mmap_sem.
+         * If no [shared/vma] mempolicy exists at the addr, get_policy() op
+         * must return NULL--i.e., do not "fallback" to task or system default
+         * policy.
+         */
         struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
                                         unsigned long addr);
         int (*migrate)(struct vm_area_struct *vma, const nodemask_t *from,