Re: [PATCH 03/18] xfs: simplify xfs_bmap_punch_delalloc_range

[Brian Foster <bfoster@redhat.com>]

On Wed, May 30, 2018 at 11:59:58AM +0200, Christoph Hellwig wrote:
> Instead of using xfs_bmapi_read to find delalloc extents and then punch
> them out using xfs_bunmapi, opencode the loop to iterate over the extents
> and call xfs_bmap_del_extent_delay directly.  This both simplifies the
> code and reduces the number of extent tree lookups required.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_bmap_util.c | 84 ++++++++++++++----------------------------
>  1 file changed, 28 insertions(+), 56 deletions(-)
> 
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index 06badcbadeb4..f2b87873612d 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -695,12 +695,10 @@ xfs_getbmap(
>  }
>  
>  /*
> - * dead simple method of punching delalyed allocation blocks from a range in
> - * the inode. Walks a block at a time so will be slow, but is only executed in
> - * rare error cases so the overhead is not critical. This will always punch out
> - * both the start and end blocks, even if the ranges only partially overlap
> - * them, so it is up to the caller to ensure that partial blocks are not
> - * passed in.
> + * Dead simple method of punching delalyed allocation blocks from a range in
> + * the inode.  This will always punch out both the start and end blocks, even
> + * if the ranges only partially overlap them, so it is up to the caller to
> + * ensure that partial blocks are not passed in.
>   */
>  int
>  xfs_bmap_punch_delalloc_range(
> @@ -708,63 +706,37 @@ xfs_bmap_punch_delalloc_range(
>  	xfs_fileoff_t		start_fsb,
>  	xfs_fileoff_t		length)
>  {
...
> +	if (!xfs_iext_lookup_extent_before(ip, ifp, &end_fsb, &icur, &got))
> +		return 0;
>  
> -		/*
> -		 * Note: while we initialise the firstblock/dfops pair, they
> -		 * should never be used because blocks should never be
> -		 * allocated or freed for a delalloc extent and hence we need
> -		 * don't cancel or finish them after the xfs_bunmapi() call.
> -		 */
> -		xfs_defer_init(&dfops, &firstblock);
> -		error = xfs_bunmapi(NULL, ip, start_fsb, 1, 0, 1, &firstblock,
> -					&dfops, &done);
> -		if (error)
> -			break;
> +	while (got.br_startoff + got.br_blockcount > start_fsb) {
> +		del = got;
> +		xfs_trim_extent(&del, start_fsb, length);
>  
> -		ASSERT(!xfs_defer_has_unfinished_work(&dfops));
> -next_block:
> -		start_fsb++;
> -		remaining--;
> -	} while(remaining > 0);
> +		if (del.br_blockcount && isnullstartblock(del.br_startblock)) {

I think there's subtle behavior here that warrants a comment (and
describes the somewhat funky logic). E.g., something like:

/*
 * got might point to the extent after del in some cases. The next
 * iteration will detect this and step back to the previous extent.
 */

Alternatively, I find separating the if/else a bit more readable (see
the appended hunk). But otherwise looks fine:

Reviewed-by: Brian Foster <bfoster@redhat.com>

> +			error = xfs_bmap_del_extent_delay(ip, XFS_DATA_FORK,
> +					&icur, &got, &del);
> +			if (error || !xfs_iext_get_extent(ifp, &icur, &got))
> +				break;
> +		} else {
> +			if (!xfs_iext_prev_extent(ifp, &icur, &got))
> +				break;
> +		}
> +	}
>  
>  	return error;
>  }
> -- 
> 2.17.0

--- 8< ---

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index f2b87873612d..0070b877ed94 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -727,15 +727,22 @@ xfs_bmap_punch_delalloc_range(
 		del = got;
 		xfs_trim_extent(&del, start_fsb, length);
 
-		if (del.br_blockcount && isnullstartblock(del.br_startblock)) {
-			error = xfs_bmap_del_extent_delay(ip, XFS_DATA_FORK,
-					&icur, &got, &del);
-			if (error || !xfs_iext_get_extent(ifp, &icur, &got))
-				break;
-		} else {
+		/*
+		 * A delete can push the cursor forward. Step back to the
+		 * previous extent on non-delalloc or extents outside the
+		 * target range.
+		 */
+		if (!del.br_blockcount ||
+		    !isnullstartblock(del.br_startblock)) {
 			if (!xfs_iext_prev_extent(ifp, &icur, &got))
 				break;
+			continue;
 		}
+
+		error = xfs_bmap_del_extent_delay(ip, XFS_DATA_FORK, &icur,
+						  &got, &del);
+		if (error || !xfs_iext_get_extent(ifp, &icur, &got))
+			break;
 	}
 
 	return error;