[PATCH] xfs: handle racy AIO in xfs_reflink_end_cow

[PATCH] xfs: handle racy AIO in xfs_reflink_end_cow

[Christoph Hellwig]

If we got two AIO writes into a COW area the second one might not have any
COW extents left to convert.  Handle that case gracefully instead of
triggering an assert or accessing beyond the bounds of the extent list.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_reflink.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 3246815c24d6..37e603bf1591 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -736,7 +736,13 @@ xfs_reflink_end_cow(
 	/* If there is a hole at end_fsb - 1 go to the previous extent */
 	if (!xfs_iext_lookup_extent(ip, ifp, end_fsb - 1, &idx, &got) ||
 	    got.br_startoff > end_fsb) {
-		ASSERT(idx > 0);
+		/*
+		 * In case of racing, overlapping AIO writes no COW extents
+		 * might be left by the time I/O completes for the loser of
+		 * the race.  In that case we are done.
+		 */
+		if (idx <= 0)
+			goto out_cancel;
 		xfs_iext_get_extent(ifp, --idx, &got);
 	}
 
@@ -809,6 +815,7 @@ xfs_reflink_end_cow(
 
 out_defer:
 	xfs_defer_cancel(&dfops);
+out_cancel:
 	xfs_trans_cancel(tp);
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 out:
-- 
2.14.1

Re: [PATCH] xfs: handle racy AIO in xfs_reflink_end_cow

[Darrick J. Wong]

On Tue, Oct 03, 2017 at 04:34:16PM +0200, Christoph Hellwig wrote:
> If we got two AIO writes into a COW area the second one might not have any
> COW extents left to convert.  Handle that case gracefully instead of
> triggering an assert or accessing beyond the bounds of the extent list.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks ok, will test.
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

> ---
>  fs/xfs/xfs_reflink.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index 3246815c24d6..37e603bf1591 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -736,7 +736,13 @@ xfs_reflink_end_cow(
>  	/* If there is a hole at end_fsb - 1 go to the previous extent */
>  	if (!xfs_iext_lookup_extent(ip, ifp, end_fsb - 1, &idx, &got) ||
>  	    got.br_startoff > end_fsb) {
> -		ASSERT(idx > 0);
> +		/*
> +		 * In case of racing, overlapping AIO writes no COW extents
> +		 * might be left by the time I/O completes for the loser of
> +		 * the race.  In that case we are done.
> +		 */
> +		if (idx <= 0)
> +			goto out_cancel;
>  		xfs_iext_get_extent(ifp, --idx, &got);
>  	}
>  
> @@ -809,6 +815,7 @@ xfs_reflink_end_cow(
>  
>  out_defer:
>  	xfs_defer_cancel(&dfops);
> +out_cancel:
>  	xfs_trans_cancel(tp);
>  	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>  out:
> -- 
> 2.14.1
> 
> --
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