Commit 487caeef authored by Jan Kara's avatar Jan Kara Committed by Theodore Ts'o
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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

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 <>.

Signed-off-by: default avatarJan Kara <>
Signed-off-by: default avatar"Theodore Ts'o" <>
parent 9599b0e5
......@@ -1370,6 +1370,7 @@ extern int ext4_change_inode_journal_flag(struct inode *, int);
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);
......@@ -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.
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;
......@@ -192,11 +192,24 @@ static int try_to_extend_transaction(handle_t *handle, struct inode *inode)
* 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.
jbd_debug(2, "restarting handle %p\n", handle);
return ext4_journal_restart(handle, blocks_for_truncate(inode));
ret = ext4_journal_restart(handle, blocks_for_truncate(inode));
return ret;
......@@ -3658,7 +3671,8 @@ static void ext4_clear_blocks(handle_t *handle, struct inode *inode,
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,
if (bh) {
BUFFER_TRACE(bh, "retaking write access");
ext4_journal_get_write_access(handle, bh);
......@@ -3869,7 +3883,8 @@ static void ext4_free_branches(handle_t *handle, struct inode *inode,
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,
ext4_free_blocks(handle, inode, nr, 1, 1);
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