linux-ext4 - Re: [PATCH 2/2] jbd: fix fsync() tid wraparound bug

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Date:	Mon, 2 May 2011 17:07:58 +0200
From:	Jan Kara <jack@...e.cz>
To:	Ted Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Martin_Zielinski@...fee.com
Subject: Re: [PATCH 2/2] jbd: fix fsync() tid wraparound bug

  Hi Ted,

On Sat 30-04-11 13:17:11, Ted Tso wrote:
> I don't know if you've been following this thread, but I was wondering
> if you could review this patch, (a) for inclusion in the ext3 tree,
> and (b) because I'd appreciate a second pair of eyes looking at this
> patch, since I intend to push similar change to jbd2.
  Thanks for forwarding. For some reason I got unsubscribed from linux-ext4
a while ago and didn't notice this since linux-fsdevel goes into the same
mailbox.

> I'm not entirely convinced this is caused by tid's wrapping around,
> since that would be a huge number of commits, but if it's not that,
> somehow i_datasync_tid or i_sync_tid is either getting corrupted or
> not getting set --- and I have no idea how that could be happening.
> This patch should at least avoid the system from crashing when we hit
> the case, and harmlessly handle the situation --- with at the worst
> case, an journal commit that wouldn't otherwise be needed.
  The patch looks OK in any case. I'll take it in my tree. It would take
about 24 days of constant 1000 trans/s load to trigger this. That's a quite
heavy load but not so unrealistic with today's HW.

> As background, I've been on this bug for months now, as it's been
> reported to me as occasionally happening on Android devices that have
> been using ext4.  Since I hadn't seen any reports of this in the field
> in the x86 world, and this code hadn't changed in a long, long time, I
> had originally assumed it was an ARM-specific bug.  However, recently,
> Martin Zielinski (on this thread) has reported this problem on an x86
> system --- and on a x86 system to boot.
> 
> Martin suspects it may have to do with sqllite --- which is consistent
> with I've seen, since I believe Android devices use sqllite quite
> heavily as well.
  Yeah, it may be.

								Honza
> jbd: fix fsync() tid wraparound bug
> 
> If an application program does not make any changes to the indirect
> blocks or extent tree, i_datasync_tid will not get updated.  If there
> are enough commits (i.e., 2**31) such that tid_geq()'s calculations
> wrap, and there isn't a currently active transaction at the time of
> the fdatasync() call, this can end up triggering a BUG_ON in
> fs/jbd/commit.c:
> 
> 	J_ASSERT(journal->j_running_transaction != NULL);
> 
> It's pretty rare that this can happen, since it requires the use of
> fdatasync() plus *very* frequent and excessive use of fsync().  But
> with the right workload, it can.
> 
> We fix this by replacing the use of tid_geq() with an equality test,
> since there's only one valid transaction id that we is valid for us to
> wait until it is commited: namely, the currently running transaction
> (if it exists).
> 
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
>  fs/jbd/journal.c |   16 +++++++++++++---
>  1 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/jbd/journal.c b/fs/jbd/journal.c
> index b3713af..1b71ce6 100644
> --- a/fs/jbd/journal.c
> +++ b/fs/jbd/journal.c
> @@ -437,9 +437,12 @@ int __log_space_left(journal_t *journal)
>  int __log_start_commit(journal_t *journal, tid_t target)
>  {
>  	/*
> -	 * Are we already doing a recent enough commit?
> +	 * The only transaction we can possibly wait upon is the
> +	 * currently running transaction (if it exists).  Otherwise,
> +	 * the target tid must be an old one.
>  	 */
> -	if (!tid_geq(journal->j_commit_request, target)) {
> +	if (journal->j_running_transaction &&
> +	    journal->j_running_transaction->t_tid == target) {
>  		/*
>  		 * We want a new commit: OK, mark the request and wakeup the
>  		 * commit thread.  We do _not_ do the commit ourselves.
> @@ -451,7 +454,14 @@ int __log_start_commit(journal_t *journal, tid_t target)
>  			  journal->j_commit_sequence);
>  		wake_up(&journal->j_wait_commit);
>  		return 1;
> -	}
> +	} else if (!tid_geq(journal->j_commit_request, target))
> +		/* This should never happen, but if it does, preserve
> +		   the evidence before kjournald goes into a loop and
> +		   increments j_commit_sequence beyond all recognition. */
> +		WARN(1, "jbd: bad log_start_commit: %u %u %u %u\n",
> +		     journal->j_commit_request, journal->j_commit_sequence,
> +		     target, journal->j_running_transaction ? 
> +		     journal->j_running_transaction->t_tid : 0);
>  	return 0;
>  }
>  
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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