mm: mempolicy: Convert shared_policy mutex to spinlock

Sasha was fuzzing with trinity and reported the following problem:

  BUG: sleeping function called from invalid context at kernel/mutex.c:269
  in_atomic(): 1, irqs_disabled(): 0, pid: 6361, name: trinity-main
  2 locks held by trinity-main/6361:
   #0:  (&mm->mmap_sem){++++++}, at: [<ffffffff810aa314>] __do_page_fault+0x1e4/0x4f0
   #1:  (&(&mm->page_table_lock)->rlock){+.+...}, at: [<ffffffff8122f017>] handle_pte_fault+0x3f7/0x6a0
  Pid: 6361, comm: trinity-main Tainted: G        W
  3.7.0-rc2-next-20121024-sasha-00001-gd95ef01-dirty #74
  Call Trace:
    __might_sleep+0x1c3/0x1e0
    mutex_lock_nested+0x29/0x50
    mpol_shared_policy_lookup+0x2e/0x90
    shmem_get_policy+0x2e/0x30
    get_vma_policy+0x5a/0xa0
    mpol_misplaced+0x41/0x1d0
    handle_pte_fault+0x465/0x6a0

This was triggered by a different version of automatic NUMA balancing
but in theory the current version is vunerable to the same problem.

do_numa_page
  -> numa_migrate_prep
    -> mpol_misplaced
      -> get_vma_policy
        -> shmem_get_policy

It's very unlikely this will happen as shared pages are not marked
pte_numa -- see the page_mapcount() check in change_pte_range() -- but
it is possible.

To address this, this patch restores sp->lock as originally implemented
by Kosaki Motohiro.  In the path where get_vma_policy() is called, it
should not be calling sp_alloc() so it is not necessary to treat the PTL
specially.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 49 insertions, 21 deletions

diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index 92bc9988a180..0d7df39a5885 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -123,7 +123,7 @@ struct sp_node {
 
 struct shared_policy {
         struct rb_root root;
-        struct mutex mutex;
+        spinlock_t lock;
 };
 
 void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 1cb200af3828..e2df1c1fb41f 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2132,7 +2132,7 @@ bool __mpol_equal(struct mempolicy *a, struct mempolicy *b)
  */
 
 /* lookup first element intersecting start-end */
-/* Caller holds sp->mutex */
+/* Caller holds sp->lock */
 static struct sp_node *
 sp_lookup(struct shared_policy *sp, unsigned long start, unsigned long end)
 {
@@ -2196,13 +2196,13 @@ mpol_shared_policy_lookup(struct shared_policy *sp, unsigned long idx)
 
         if (!sp->root.rb_node)
                 return NULL;
-        mutex_lock(&sp->mutex);
+        spin_lock(&sp->lock);
         sn = sp_lookup(sp, idx, idx+1);
         if (sn) {
                 mpol_get(sn->policy);
                 pol = sn->policy;
         }
-        mutex_unlock(&sp->mutex);
+        spin_unlock(&sp->lock);
         return pol;
 }
 
@@ -2328,6 +2328,14 @@ static void sp_delete(struct shared_policy *sp, struct sp_node *n)
         sp_free(n);
 }
 
+static void sp_node_init(struct sp_node *node, unsigned long start,
+                        unsigned long end, struct mempolicy *pol)
+{
+        node->start = start;
+        node->end = end;
+        node->policy = pol;
+}
+
 static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
                                 struct mempolicy *pol)
 {
@@ -2344,10 +2352,7 @@ static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
                 return NULL;
         }
         newpol->flags |= MPOL_F_SHARED;
-
-        n->start = start;
-        n->end = end;
-        n->policy = newpol;
+        sp_node_init(n, start, end, newpol);
 
         return n;
 }
@@ -2357,9 +2362,12 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
                                  unsigned long end, struct sp_node *new)
 {
         struct sp_node *n;
+        struct sp_node *n_new = NULL;
+        struct mempolicy *mpol_new = NULL;
         int ret = 0;
 
-        mutex_lock(&sp->mutex);
+restart:
+        spin_lock(&sp->lock);
         n = sp_lookup(sp, start, end);
         /* Take care of old policies in the same range. */
         while (n && n->start < end) {
@@ -2372,14 +2380,16 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
                 } else {
                         /* Old policy spanning whole new range. */
                         if (n->end > end) {
-                                struct sp_node *new2;
-                                new2 = sp_alloc(end, n->end, n->policy);
-                                if (!new2) {
-                                        ret = -ENOMEM;
-                                        goto out;
-                                }
+                                if (!n_new)
+                                        goto alloc_new;
+
+                                *mpol_new = *n->policy;
+                                atomic_set(&mpol_new->refcnt, 1);
+                                sp_node_init(n_new, n->end, end, mpol_new);
+                                sp_insert(sp, n_new);
                                 n->end = start;
-                                sp_insert(sp, new2);
+                                n_new = NULL;
+                                mpol_new = NULL;
                                 break;
                         } else
                                 n->end = start;
@@ -2390,9 +2400,27 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
         }
         if (new)
                 sp_insert(sp, new);
-out:
-        mutex_unlock(&sp->mutex);
+        spin_unlock(&sp->lock);
+        ret = 0;
+
+err_out:
+        if (mpol_new)
+                mpol_put(mpol_new);
+        if (n_new)
+                kmem_cache_free(sn_cache, n_new);
+
         return ret;
+
+alloc_new:
+        spin_unlock(&sp->lock);
+        ret = -ENOMEM;
+        n_new = kmem_cache_alloc(sn_cache, GFP_KERNEL);
+        if (!n_new)
+                goto err_out;
+        mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
+        if (!mpol_new)
+                goto err_out;
+        goto restart;
 }
 
 /**
@@ -2410,7 +2438,7 @@ void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol)
         int ret;
 
         sp->root = RB_ROOT;             /* empty tree == default mempolicy */
-        mutex_init(&sp->mutex);
+        spin_lock_init(&sp->lock);
 
         if (mpol) {
                 struct vm_area_struct pvma;
@@ -2476,14 +2504,14 @@ void mpol_free_shared_policy(struct shared_policy *p)
 
         if (!p->root.rb_node)
                 return;
-        mutex_lock(&p->mutex);
+        spin_lock(&p->lock);
         next = rb_first(&p->root);
         while (next) {
                 n = rb_entry(next, struct sp_node, nd);
                 next = rb_next(&n->nd);
                 sp_delete(p, n);
         }
-        mutex_unlock(&p->mutex);
+        spin_unlock(&p->lock);
 }
 
 #ifdef CONFIG_NUMA_BALANCING