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Date: Tue, 16 Jun 2015 15:29:20 -0400
From: Eric Whitney <enwlinux@...il.com>
To: Eric Whitney <enwlinux@...il.com>
Cc: Theodore Ts'o <tytso@....edu>,
Ext4 Developers List <linux-ext4@...r.kernel.org>,
jack@...e.cz, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix race between truncate and
__ext4_journalled_writepage()
* Eric Whitney <enwlinux@...il.com>:
> * Theodore Ts'o <tytso@....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@....edu>
> > Cc: stable@...r.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
> >
>
> This patch looks promising. I'm running a 1000 trial stress test on a
> Pandaboard where I've generally been able to force a couple of manifestations
> of this bug to appear within 5 to 10 runs. Applied to 4.1-rc7, it's passed
> 135 trials cleanly. The full series will complete sometime tomorrow.
>
> I was also able to reproduce the problem on x86_64 pretty consistently in
> four runs or less on 4.1-rc7; I'm planning a stress test there as well once
> -rc8 regression is complete.
>
1000 consecutive runs of generic/068 on the data_journal test case have
completed successfully on an x86_64 guest and a Pandaboard running a
4.1-rc7 kernel with this patch. That's a couple of orders of magnitude more
runs than have previously completed without hangs on these test system
configurations, so
Tested-by: Eric Whitney <enwlinux@...il.com>
if useful at this point.
Eric
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