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>
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 2e9a203..fb21663 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1370,6 +1370,7 @@
extern int ext4_get_inode_loc(struct inode *, struct ext4_iloc *);
extern int ext4_can_truncate(struct inode *inode);
extern void ext4_truncate(struct inode *);
+extern int ext4_truncate_restart_trans(handle_t *, struct inode *, int nblocks);
extern void ext4_set_inode_flags(struct inode *);
extern void ext4_get_inode_flags(struct ext4_inode_info *);
extern int ext4_alloc_da_blocks(struct inode *inode);
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 13db434..8c20caf 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -93,7 +93,9 @@
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 @@
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 @@
}
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;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 9a4c929..d61fb52 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -192,11 +192,24 @@
* so before we call here everything must be consistently dirtied against
* this transaction.
*/
-static int ext4_journal_test_restart(handle_t *handle, struct inode *inode)
+ int ext4_truncate_restart_trans(handle_t *handle, struct inode *inode,
+ int nblocks)
{
+ int ret;
+
+ /*
+ * Drop i_data_sem to avoid deadlock with ext4_get_blocks At this
+ * moment, get_block can be called only for blocks inside i_size since
+ * page cache has been already dropped and writes are blocked by
+ * i_mutex. So we can safely drop the i_data_sem here.
+ */
BUG_ON(EXT4_JOURNAL(inode) == NULL);
jbd_debug(2, "restarting handle %p\n", handle);
- return ext4_journal_restart(handle, blocks_for_truncate(inode));
+ up_write(&EXT4_I(inode)->i_data_sem);
+ ret = ext4_journal_restart(handle, blocks_for_truncate(inode));
+ down_write(&EXT4_I(inode)->i_data_sem);
+
+ return ret;
}
/*
@@ -3658,7 +3671,8 @@
ext4_handle_dirty_metadata(handle, inode, bh);
}
ext4_mark_inode_dirty(handle, inode);
- ext4_journal_test_restart(handle, inode);
+ ext4_truncate_restart_trans(handle, inode,
+ blocks_for_truncate(inode));
if (bh) {
BUFFER_TRACE(bh, "retaking write access");
ext4_journal_get_write_access(handle, bh);
@@ -3869,7 +3883,8 @@
return;
if (try_to_extend_transaction(handle, inode)) {
ext4_mark_inode_dirty(handle, inode);
- ext4_journal_test_restart(handle, inode);
+ ext4_truncate_restart_trans(handle, inode,
+ blocks_for_truncate(inode));
}
ext4_free_blocks(handle, inode, nr, 1, 1);