1 files changed, 29 insertions, 12 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 3f696f7d9bac..294364a24a1f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -264,6 +264,24 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
 }
 
 /*
+ * Sometimes, before we decide whether to proceed or to fail, we must check
+ * that an entry was not already brought back from swap by a racing thread.
+ *
+ * Checking page is not enough: by the time a SwapCache page is locked, it
+ * might be reused, and again be SwapCache, using the same swap as before.
+ */
+static bool shmem_confirm_swap(struct address_space *mapping,
+                               pgoff_t index, swp_entry_t swap)
+{
+        void *item;
+
+        rcu_read_lock();
+        item = radix_tree_lookup(&mapping->page_tree, index);
+        rcu_read_unlock();
+        return item == swp_to_radix_entry(swap);
+}
+
+/*
  * Like add_to_page_cache_locked, but error if expected item has gone.
  */
 static int shmem_add_to_page_cache(struct page *page,
@@ -1124,9 +1142,9 @@ repeat:
                 /* We have to do this with page locked to prevent races */
                 lock_page(page);
                 if (!PageSwapCache(page) || page_private(page) != swap.val ||
-                    page->mapping) {
+                    !shmem_confirm_swap(mapping, index, swap)) {
                         error = -EEXIST;        /* try again */
-                        goto failed;
+                        goto unlock;
                 }
                 if (!PageUptodate(page)) {
                         error = -EIO;
@@ -1142,9 +1160,12 @@ repeat:
 
                 error = mem_cgroup_cache_charge(page, current->mm,
                                                 gfp & GFP_RECLAIM_MASK);
-                if (!error)
+                if (!error) {
                         error = shmem_add_to_page_cache(page, mapping, index,
                                                 gfp, swp_to_radix_entry(swap));
+                        /* We already confirmed swap, and make no allocation */
+                        VM_BUG_ON(error);
+                }
                 if (error)
                         goto failed;
 
@@ -1245,14 +1266,10 @@ decused:
 unacct:
         shmem_unacct_blocks(info->flags, 1);
 failed:
-        if (swap.val && error != -EINVAL) {
-                struct page *test = find_get_page(mapping, index);
-                if (test && !radix_tree_exceptional_entry(test))
-                        page_cache_release(test);
-                /* Have another try if the entry has changed */
-                if (test != swp_to_radix_entry(swap))
-                        error = -EEXIST;
-        }
+        if (swap.val && error != -EINVAL &&
+            !shmem_confirm_swap(mapping, index, swap))
+                error = -EEXIST;
+unlock:
         if (page) {
                 unlock_page(page);
                 page_cache_release(page);
@@ -1264,7 +1281,7 @@ failed:
                 spin_unlock(&info->lock);
                 goto repeat;
         }
-        if (error == -EEXIST)
+        if (error == -EEXIST)   /* from above or from radix_tree_insert */
                 goto repeat;
         return error;
 }