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

1 files changed, 20 insertions, 4 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 58dede76f75f..3dbc56eb4849 100644
--- a/fs/ext4/ext4.h
+++ b/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 {