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

Brian Foster
2018-05-30T13:35:41Z

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;