ext4: fix lost truncate due to race with writeback

The following race can lead to a loss of i_disksize update from truncate
thus resulting in a wrong inode size if the inode size isn't updated
again before inode is reclaimed:

ext4_setattr()				mpage_map_and_submit_extent()
  EXT4_I(inode)->i_disksize = attr->ia_size;
  ...					  ...
					  disksize = ((loff_t)mpd->first_page) << PAGE_CACHE_SHIFT
					  /* False because i_size isn't
					   * updated yet */
					  if (disksize > i_size_read(inode))
					  /* True, because i_disksize is
					   * already truncated */
					  if (disksize > EXT4_I(inode)->i_disksize)
					    /* Overwrite i_disksize
					     * update from truncate */
					    ext4_update_i_disksize()
  i_size_write(inode, attr->ia_size);

For other places updating i_disksize such race cannot happen because
i_mutex prevents these races. Writeback is the only place where we do
not hold i_mutex and we cannot grab it there because of lock ordering.

We fix the race by doing both i_disksize and i_size update in truncate
atomically under i_data_sem and in mpage_map_and_submit_extent() we move
the check against i_size under i_data_sem as well.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/ext4.h

@@ -2432,16 +2432,32 @@ do {								\
 #define EXT4_FREECLUSTERS_WATERMARK 0
 #endif
 
+/* Update i_disksize. Requires i_mutex to avoid races with truncate */
 static inline void ext4_update_i_disksize(struct inode *inode, loff_t newsize)
 {
-	/*
-	 * XXX: replace with spinlock if seen contended -bzzz
-	 */
+	WARN_ON_ONCE(S_ISREG(inode->i_mode) &&
+		     !mutex_is_locked(&inode->i_mutex));
+	down_write(&EXT4_I(inode)->i_data_sem);
+	if (newsize > EXT4_I(inode)->i_disksize)
+		EXT4_I(inode)->i_disksize = newsize;
+	up_write(&EXT4_I(inode)->i_data_sem);
+}
+
+/*
+ * Update i_disksize after writeback has been started. Races with truncate
+ * are avoided by checking i_size under i_data_sem.
+ */
+static inline void ext4_wb_update_i_disksize(struct inode *inode, loff_t newsize)
+{
+	loff_t i_size;
+
 	down_write(&EXT4_I(inode)->i_data_sem);
+	i_size = i_size_read(inode);
+	if (newsize > i_size)
+		newsize = i_size;
 	if (newsize > EXT4_I(inode)->i_disksize)
 		EXT4_I(inode)->i_disksize = newsize;
 	up_write(&EXT4_I(inode)->i_data_sem);
-	return ;
 }
 
 struct ext4_group_info {

fs/ext4/inode.c

@@ -2237,12 +2237,10 @@ static int mpage_map_and_submit_extent(handle_t *handle,
 
 	/* Update on-disk size after IO is submitted */
 	disksize = ((loff_t)mpd->first_page) << PAGE_CACHE_SHIFT;
-	if (disksize > i_size_read(inode))
-		disksize = i_size_read(inode);
 	if (disksize > EXT4_I(inode)->i_disksize) {
 		int err2;
 
-		ext4_update_i_disksize(inode, disksize);
+		ext4_wb_update_i_disksize(inode, disksize);
 		err2 = ext4_mark_inode_dirty(handle, inode);
 		if (err2)
 			ext4_error(inode->i_sb,
@@ -4627,18 +4625,27 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
 				error = ext4_orphan_add(handle, inode);
 				orphan = 1;
 			}
+			down_write(&EXT4_I(inode)->i_data_sem);
 			EXT4_I(inode)->i_disksize = attr->ia_size;
 			rc = ext4_mark_inode_dirty(handle, inode);
 			if (!error)
 				error = rc;
+			/*
+			 * We have to update i_size under i_data_sem together
+			 * with i_disksize to avoid races with writeback code
+			 * running ext4_wb_update_i_disksize().
+			 */
+			if (!error)
+				i_size_write(inode, attr->ia_size);
+			up_write(&EXT4_I(inode)->i_data_sem);
 			ext4_journal_stop(handle);
 			if (error) {
 				ext4_orphan_del(NULL, inode);
 				goto err_out;
 			}
-		}
+		} else
+			i_size_write(inode, attr->ia_size);
 
-		i_size_write(inode, attr->ia_size);
 		/*
 		 * Blocks are going to be removed from the inode. Wait
 		 * for dio in flight.  Temporarily disable