ext4: Fix possible deadlock between ext4_truncate() and ext4_get_blocks()

During truncate we are sometimes forced to start a new transaction as
the amount of blocks to be journaled is both quite large and hard to
predict. So far we restarted a transaction while holding i_data_sem
and that violates lock ordering because i_data_sem ranks below a
transaction start (and it can lead to a real deadlock with
ext4_get_blocks() mapping blocks in some page while having a
transaction open).

We fix the problem by dropping the i_data_sem before restarting the
transaction and acquire it afterwards. It's slightly subtle that this
works:

1) By the time ext4_truncate() is called, all the page cache for the
truncated part of the file is dropped so get_block() should not be
called on it (we only have to invalidate extent cache after we
reacquire i_data_sem because some extent from not-truncated part could
extend also into the part we are going to truncate).

2) Writes, migrate or defrag hold i_mutex so they are stopped for all
the time of the truncate.

This bug has been found and analyzed by Theodore Tso <tytso@mit.edu>.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 12 insertions, 3 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 13db43408533..8c20caf4aa5c 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -93,7 +93,9 @@ static void ext4_idx_store_pblock(struct ext4_extent_idx *ix, ext4_fsblk_t pb)
 	ix->ei_leaf_hi = cpu_to_le16((unsigned long) ((pb >> 31) >> 1) & 0xffff);
 }
 
-static int ext4_ext_journal_restart(handle_t *handle, int needed)
+static int ext4_ext_truncate_extend_restart(handle_t *handle,
+					    struct inode *inode,
+					    int needed)
 {
 	int err;
 
@@ -104,7 +106,14 @@ static int ext4_ext_journal_restart(handle_t *handle, int needed)
 	err = ext4_journal_extend(handle, needed);
 	if (err <= 0)
 		return err;
-	return ext4_journal_restart(handle, needed);
+	err = ext4_truncate_restart_trans(handle, inode, needed);
+	/*
+	 * We have dropped i_data_sem so someone might have cached again
+	 * an extent we are going to truncate.
+	 */
+	ext4_ext_invalidate_cache(inode);
+
+	return err;
 }
 
 /*
@@ -2150,7 +2159,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
 		}
 		credits += 2 * EXT4_QUOTA_TRANS_BLOCKS(inode->i_sb);
 
-		err = ext4_ext_journal_restart(handle, credits);
+		err = ext4_ext_truncate_extend_restart(handle, inode, credits);
 		if (err)
 			goto out;