1 files changed, 37 insertions, 20 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index c244e93a70fa..4ce02e0673db 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -683,10 +683,21 @@ static int shmem_unuse_inode(struct shmem_inode_info *info,
                 mutex_lock(&shmem_swaplist_mutex);
                 /*
                  * We needed to drop mutex to make that restrictive page
-                 * allocation; but the inode might already be freed by now,
-                 * and we cannot refer to inode or mapping or info to check.
-                 * However, we do hold page lock on the PageSwapCache page,
-                 * so can check if that still has our reference remaining.
+                 * allocation, but the inode might have been freed while we
+                 * dropped it: although a racing shmem_evict_inode() cannot
+                 * complete without emptying the radix_tree, our page lock
+                 * on this swapcache page is not enough to prevent that -
+                 * free_swap_and_cache() of our swap entry will only
+                 * trylock_page(), removing swap from radix_tree whatever.
+                 *
+                 * We must not proceed to shmem_add_to_page_cache() if the
+                 * inode has been freed, but of course we cannot rely on
+                 * inode or mapping or info to check that.  However, we can
+                 * safely check if our swap entry is still in use (and here
+                 * it can't have got reused for another page): if it's still
+                 * in use, then the inode cannot have been freed yet, and we
+                 * can safely proceed (if it's no longer in use, that tells
+                 * nothing about the inode, but we don't need to unuse swap).
                  */
                 if (!page_swapcount(*pagep))
                         error = -ENOENT;
@@ -730,9 +741,9 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
 
         /*
          * There's a faint possibility that swap page was replaced before
-         * caller locked it: it will come back later with the right page.
+         * caller locked it: caller will come back later with the right page.
          */
-        if (unlikely(!PageSwapCache(page)))
+        if (unlikely(!PageSwapCache(page) || page_private(page) != swap.val))
                 goto out;
 
         /*
@@ -995,21 +1006,15 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
         newpage = shmem_alloc_page(gfp, info, index);
         if (!newpage)
                 return -ENOMEM;
-        VM_BUG_ON(shmem_should_replace_page(newpage, gfp));
 
-        *pagep = newpage;
         page_cache_get(newpage);
         copy_highpage(newpage, oldpage);
+        flush_dcache_page(newpage);
 
-        VM_BUG_ON(!PageLocked(oldpage));
         __set_page_locked(newpage);
-        VM_BUG_ON(!PageUptodate(oldpage));
         SetPageUptodate(newpage);
-        VM_BUG_ON(!PageSwapBacked(oldpage));
         SetPageSwapBacked(newpage);
-        VM_BUG_ON(!swap_index);
         set_page_private(newpage, swap_index);
-        VM_BUG_ON(!PageSwapCache(oldpage));
         SetPageSwapCache(newpage);
 
         /*
@@ -1019,13 +1024,24 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
         spin_lock_irq(&swap_mapping->tree_lock);
         error = shmem_radix_tree_replace(swap_mapping, swap_index, oldpage,
                                                                    newpage);
-        __inc_zone_page_state(newpage, NR_FILE_PAGES);
-        __dec_zone_page_state(oldpage, NR_FILE_PAGES);
+        if (!error) {
+                __inc_zone_page_state(newpage, NR_FILE_PAGES);
+                __dec_zone_page_state(oldpage, NR_FILE_PAGES);
+        }
         spin_unlock_irq(&swap_mapping->tree_lock);
-        BUG_ON(error);
 
-        mem_cgroup_replace_page_cache(oldpage, newpage);
-        lru_cache_add_anon(newpage);
+        if (unlikely(error)) {
+                /*
+                 * Is this possible?  I think not, now that our callers check
+                 * both PageSwapCache and page_private after getting page lock;
+                 * but be defensive.  Reverse old to newpage for clear and free.
+                 */
+                oldpage = newpage;
+        } else {
+                mem_cgroup_replace_page_cache(oldpage, newpage);
+                lru_cache_add_anon(newpage);
+                *pagep = newpage;
+        }
 
         ClearPageSwapCache(oldpage);
         set_page_private(oldpage, 0);
@@ -1033,7 +1049,7 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
         unlock_page(oldpage);
         page_cache_release(oldpage);
         page_cache_release(oldpage);
-        return 0;
+        return error;
 }
 
 /*
@@ -1107,7 +1123,8 @@ repeat:
 
                 /* We have to do this with page locked to prevent races */
                 lock_page(page);
-                if (!PageSwapCache(page) || page->mapping) {
+                if (!PageSwapCache(page) || page_private(page) != swap.val ||
+                    page->mapping) {
                         error = -EEXIST;        /* try again */
                         goto failed;
                 }