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

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: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent 9599b0e5

fs/ext4/ext4.h

+1 −0
Original line number Diff line number Diff line
@@ -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);

fs/ext4/extents.c

+12 −3
Original line number Diff line number Diff line
@@ -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;

fs/ext4/inode.c

+19 −4
Original line number Diff line number Diff line
@@ -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.

	 */

	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 @@ 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,

					    blocks_for_truncate(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, 
				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);

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