mempolicy: fixup Fallback for Default Shmem Policy

get_vma_policy() is not handling fallback to task policy correctly when the
get_policy() vm_op returns NULL.  The NULL overwrites the 'pol' variable that
was holding the fallback task mempolicy.  So, it was falling back directly to
system default policy.

Fix get_vma_policy() to use only non-NULL policy returned from the vma
get_policy op.

shm_get_policy() was falling back to current task's mempolicy if the "backing
file system" [tmpfs vs hugetlbfs] does not support the get_policy vm_op and
the vma policy is null.  This is incorrect for show_numa_maps() which is
likely querying the numa_maps of some task other than current.  Remove this
fallback.

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>

2 files changed, 6 insertions, 4 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index cc63fae02f06..8d1b2c468cc4 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -274,8 +274,7 @@ static struct mempolicy *shm_get_policy(struct vm_area_struct *vma,
         else if (vma->vm_policy) {
                 pol = vma->vm_policy;
                 mpol_get(pol);  /* get_vma_policy() expects this */
-        } else
-                pol = current->mempolicy;
+        }
         return pol;
 }
 #endif
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index c6c61ea6bb8c..8924aaf4665c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1262,7 +1262,7 @@ asmlinkage long compat_sys_mbind(compat_ulong_t start, compat_ulong_t len,
  * @task != current].  It is the caller's responsibility to
  * free the reference in these cases.
  */
-static struct mempolicy * get_vma_policy(struct task_struct *task,
+static struct mempolicy *get_vma_policy(struct task_struct *task,
                 struct vm_area_struct *vma, unsigned long addr)
 {
         struct mempolicy *pol = task->mempolicy;
@@ -1270,7 +1270,10 @@ static struct mempolicy * get_vma_policy(struct task_struct *task,
 
         if (vma) {
                 if (vma->vm_ops && vma->vm_ops->get_policy) {
-                        pol = vma->vm_ops->get_policy(vma, addr);
+                        struct mempolicy *vpol = vma->vm_ops->get_policy(vma,
+                                                                        addr);
+                        if (vpol)
+                                pol = vpol;
                         shared_pol = 1; /* if pol non-NULL, add ref below */
                 } else if (vma->vm_policy &&
                                 vma->vm_policy->policy != MPOL_DEFAULT)