Re: [PATCH v3 2/2] xfs: use sync buffer I/O for sync delwri queue submission

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jun 15, 2018 at 02:05:36PM -0400, Brian Foster wrote:
> If a delwri queue occurs of a buffer that sits on a delwri queue
> wait list, the queue sets _XBF_DELWRI_Q without changing the state
> of ->b_list. This occurs, for example, if another thread beats the
> current delwri waiter thread to the buffer lock after I/O
> completion. Once the waiter acquires the lock, it removes the buffer
> from the wait list and leaves a buffer with _XBF_DELWRI_Q set but
> not populated on a list. This results in a lost buffer submission
> and in turn can result in assert failures due to _XBF_DELWRI_Q being
> set on buffer reclaim or filesystem lockups if the buffer happens to
> cover an item in the AIL.
> 
> This problem has been reproduced by repeated iterations of xfs/305
> on high CPU count (28xcpu) systems with limited memory (~1GB). Dirty
> dquot reclaim races with an xfsaild push of a separate dquot backed
> by the same buffer such that the buffer sits on the reclaim wait
> list at the time xfsaild attempts to queue it. Since the latter
> dquot has been flush locked but the underlying buffer not submitted
> for I/O, the dquot pins the AIL and causes the filesystem to
> livelock.
> 
> This race is essentially made possible by the buffer lock cycle
> involved with waiting on a synchronous delwri queue submission.
> Close the race by using synchronous buffer I/O for respective delwri
> queue submission. This means the buffer remains locked across the
> I/O and so is inaccessible from other contexts while in the
> intermediate wait list state. The sync buffer I/O wait mechanism is
> factored into a helper such that sync delwri buffer submission and
> serialization are batched operations.
> 
> Designed-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/xfs_buf.c | 78 +++++++++++++++++++++++++-----------------------
>  1 file changed, 41 insertions(+), 37 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 7b0f7c79cd62..b5c0f6949b30 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1531,6 +1531,20 @@ xfs_buf_submit(
>  	xfs_buf_rele(bp);
>  }
>  
> +/*
> + * Wait for I/O completion of a sync buffer and return the I/O error code.
> + */
> +static int
> +xfs_buf_iowait(
> +	struct xfs_buf	*bp)
> +{
> +	trace_xfs_buf_iowait(bp, _RET_IP_);
> +	wait_for_completion(&bp->b_iowait);
> +	trace_xfs_buf_iowait_done(bp, _RET_IP_);
> +
> +	return bp->b_error;
> +}
> +
>  /*
>   * Synchronous buffer IO submission path, read or write.
>   */
> @@ -1553,12 +1567,7 @@ xfs_buf_submit_wait(
>  	error = __xfs_buf_submit(bp);
>  	if (error)
>  		goto out;
> -
> -	/* wait for completion before gathering the error from the buffer */
> -	trace_xfs_buf_iowait(bp, _RET_IP_);
> -	wait_for_completion(&bp->b_iowait);
> -	trace_xfs_buf_iowait_done(bp, _RET_IP_);
> -	error = bp->b_error;
> +	error = xfs_buf_iowait(bp);
>  
>  out:
>  	/*
> @@ -1961,16 +1970,11 @@ xfs_buf_cmp(
>  }
>  
>  /*
> - * submit buffers for write.
> - *
> - * When we have a large buffer list, we do not want to hold all the buffers
> - * locked while we block on the request queue waiting for IO dispatch. To avoid
> - * this problem, we lock and submit buffers in groups of 50, thereby minimising
> - * the lock hold times for lists which may contain thousands of objects.
> - *
> - * To do this, we sort the buffer list before we walk the list to lock and
> - * submit buffers, and we plug and unplug around each group of buffers we
> - * submit.
> + * Submit buffers for write. If wait_list is specified, the buffers are
> + * submitted using sync I/O and placed on the wait list such that the caller can
> + * iowait each buffer. Otherwise async I/O is used and the buffers are released
> + * at I/O completion time. In either case, buffers remain locked until I/O
> + * completes and the buffer is released from the queue.
>   */
>  static int
>  xfs_buf_delwri_submit_buffers(
> @@ -2012,21 +2016,22 @@ xfs_buf_delwri_submit_buffers(
>  		trace_xfs_buf_delwri_split(bp, _RET_IP_);
>  
>  		/*
> -		 * We do all IO submission async. This means if we need
> -		 * to wait for IO completion we need to take an extra
> -		 * reference so the buffer is still valid on the other
> -		 * side. We need to move the buffer onto the io_list
> -		 * at this point so the caller can still access it.
> +		 * If we have a wait list, each buffer (and associated delwri
> +		 * queue reference) transfers to it and is submitted
> +		 * synchronously. Otherwise, drop the buffer from the delwri
> +		 * queue and submit async.
>  		 */
>  		bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_WRITE_FAIL);
> -		bp->b_flags |= XBF_WRITE | XBF_ASYNC;
> +		bp->b_flags |= XBF_WRITE;
>  		if (wait_list) {
> -			xfs_buf_hold(bp);
> +			bp->b_flags &= ~XBF_ASYNC;
>  			list_move_tail(&bp->b_list, wait_list);
> -		} else
> +			__xfs_buf_submit(bp);
> +		} else {
> +			bp->b_flags |= XBF_ASYNC;
>  			list_del_init(&bp->b_list);
> -
> -		xfs_buf_submit(bp);
> +			xfs_buf_submit(bp);
> +		}
>  	}
>  	blk_finish_plug(&plug);
>  
> @@ -2073,8 +2078,11 @@ xfs_buf_delwri_submit(
>  
>  		list_del_init(&bp->b_list);
>  
> -		/* locking the buffer will wait for async IO completion. */
> -		xfs_buf_lock(bp);
> +		/*
> +		 * Wait on the locked buffer, check for errors and unlock and
> +		 * release the delwri queue reference.
> +		 */
> +		xfs_buf_iowait(bp);
>  		error2 = bp->b_error;

error2 = xfs_buf_iowait()?

>  		xfs_buf_relse(bp);
>  		if (!error)
> @@ -2121,22 +2129,18 @@ xfs_buf_delwri_pushbuf(
>  
>  	/*
>  	 * Delwri submission clears the DELWRI_Q buffer flag and returns with
> -	 * the buffer on the wait list with an associated reference. Rather than
> +	 * the buffer on the wait list with the original reference. Rather than
>  	 * bounce the buffer from a local wait list back to the original list
>  	 * after I/O completion, reuse the original list as the wait list.
>  	 */
>  	xfs_buf_delwri_submit_buffers(&submit_list, buffer_list);
>  
>  	/*
> -	 * The buffer is now under I/O and wait listed as during typical delwri
> -	 * submission. Lock the buffer to wait for I/O completion. Rather than
> -	 * remove the buffer from the wait list and release the reference, we
> -	 * want to return with the buffer queued to the original list. The
> -	 * buffer already sits on the original list with a wait list reference,
> -	 * however. If we let the queue inherit that wait list reference, all we
> -	 * need to do is reset the DELWRI_Q flag.
> +	 * The buffer is now locked, under I/O and wait listed on the original
> +	 * delwri queue. Wait for I/O completion, restore the DELWRI_Q flag and
> +	 * return with the buffer unlocked and on the original queue.
>  	 */
> -	xfs_buf_lock(bp);
> +	xfs_buf_iowait(bp);
>  	error = bp->b_error;

error = xfs_buf_iowait()?

Or is there a specific reason you collected the error separately?

Looks reasonable and I could follow along this time. :)

--D

>  	bp->b_flags |= _XBF_DELWRI_Q;
>  	xfs_buf_unlock(bp);
> -- 
> 2.17.1
> 
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