[PATCH] ext4: fix race between truncate and __ext4_journalled_writepage()

[Theodore Ts'o <tytso@mit.edu>]

The commit cf108bca465d: "ext4: Invert the locking order of page_lock
and transaction start" caused __ext4_journalled_writepage() to drop
the page lock before the page was written back, as part of changing
the locking order to jbd2_journal_start -> page_lock.  However, this
introduced a potential race if there was a truncate racing with the
data=journalled writeback mode.

Fix this by grabbing the page lock after starting the journal handle,
and then checking to see if page had gotten truncated out from under
us.

This fixes a number of different crashes or BUG_ON's when running
xfstests generic/086 in data=journalled mode, including:

jbd2_journal_dirty_metadata: vdc-8: bad jh for block 84434: transaction (ec90434
ransaction (  (null), 0), jh->b_next_transaction (  (null), 0), jlist 0

	      	      	  - and -

kernel BUG at /usr/projects/linux/ext4/fs/jbd2/transaction.c:2200!
    ...
Call Trace:
 [<c02b2ded>] ? __ext4_journalled_invalidatepage+0x117/0x117
 [<c02b2de5>] __ext4_journalled_invalidatepage+0x10f/0x117
 [<c02b2ded>] ? __ext4_journalled_invalidatepage+0x117/0x117
 [<c027d883>] ? lock_buffer+0x36/0x36
 [<c02b2dfa>] ext4_journalled_invalidatepage+0xd/0x22
 [<c0229139>] do_invalidatepage+0x22/0x26
 [<c0229198>] truncate_inode_page+0x5b/0x85
 [<c022934b>] truncate_inode_pages_range+0x156/0x38c
 [<c0229592>] truncate_inode_pages+0x11/0x15
 [<c022962d>] truncate_pagecache+0x55/0x71
 [<c02b913b>] ext4_setattr+0x4a9/0x560
 [<c01ca542>] ? current_kernel_time+0x10/0x44
 [<c026c4d8>] notify_change+0x1c7/0x2be
 [<c0256a00>] do_truncate+0x65/0x85
 [<c0226f31>] ? file_ra_state_init+0x12/0x29

	      	      	  - and -

WARNING: CPU: 1 PID: 1331 at /usr/projects/linux/ext4/fs/jbd2/transaction.c:1396
irty_metadata+0x14a/0x1ae()
    ...
Call Trace:
 [<c01b879f>] ? console_unlock+0x3a1/0x3ce
 [<c082cbb4>] dump_stack+0x48/0x60
 [<c0178b65>] warn_slowpath_common+0x89/0xa0
 [<c02ef2cf>] ? jbd2_journal_dirty_metadata+0x14a/0x1ae
 [<c0178bef>] warn_slowpath_null+0x14/0x18
 [<c02ef2cf>] jbd2_journal_dirty_metadata+0x14a/0x1ae
 [<c02d8615>] __ext4_handle_dirty_metadata+0xd4/0x19d
 [<c02b2f44>] write_end_fn+0x40/0x53
 [<c02b4a16>] ext4_walk_page_buffers+0x4e/0x6a
 [<c02b59e7>] ext4_writepage+0x354/0x3b8
 [<c02b2f04>] ? mpage_release_unused_pages+0xd4/0xd4
 [<c02b1b21>] ? wait_on_buffer+0x2c/0x2c
 [<c02b5a4b>] ? ext4_writepage+0x3b8/0x3b8
 [<c02b5a5b>] __writepage+0x10/0x2e
 [<c0225956>] write_cache_pages+0x22d/0x32c
 [<c02b5a4b>] ? ext4_writepage+0x3b8/0x3b8
 [<c02b6ee8>] ext4_writepages+0x102/0x607
 [<c019adfe>] ? sched_clock_local+0x10/0x10e
 [<c01a8a7c>] ? __lock_is_held+0x2e/0x44
 [<c01a8ad5>] ? lock_is_held+0x43/0x51
 [<c0226dff>] do_writepages+0x1c/0x29
 [<c0276bed>] __writeback_single_inode+0xc3/0x545
 [<c0277c07>] writeback_sb_inodes+0x21f/0x36d
    ...

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
---
 fs/ext4/inode.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0554b0b..263a46c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1701,19 +1701,32 @@ static int __ext4_journalled_writepage(struct page *page,
 		ext4_walk_page_buffers(handle, page_bufs, 0, len,
 				       NULL, bget_one);
 	}
-	/* As soon as we unlock the page, it can go away, but we have
-	 * references to buffers so we are safe */
+	/*
+	 * We need to release the page lock before we start the
+	 * journal, so grab a reference so the page won't disappear
+	 * out from under us.
+	 */
+	get_page(page);
 	unlock_page(page);
 
 	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE,
 				    ext4_writepage_trans_blocks(inode));
 	if (IS_ERR(handle)) {
 		ret = PTR_ERR(handle);
-		goto out;
+		put_page(page);
+		goto out_no_pagelock;
 	}
-
 	BUG_ON(!ext4_handle_valid(handle));
 
+	lock_page(page);
+	put_page(page);
+	if (page->mapping != mapping) {
+		/* The page got truncated from under us */
+		ext4_journal_stop(handle);
+		ret = 0;
+		goto out;
+	}
+
 	if (inline_data) {
 		BUFFER_TRACE(inode_bh, "get write access");
 		ret = ext4_journal_get_write_access(handle, inode_bh);
@@ -1739,6 +1752,8 @@ static int __ext4_journalled_writepage(struct page *page,
 				       NULL, bput_one);
 	ext4_set_inode_state(inode, EXT4_STATE_JDATA);
 out:
+	unlock_page(page);
+out_no_pagelock:
 	brelse(inode_bh);
 	return ret;
 }
-- 
2.3.0