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

Darrick J. Wong
2018-05-30T16:56:06Z

On Wed, May 30, 2018 at 09:35:39AM -0400, Brian Foster wrote:
> 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>

/me agrees (and adds the hunk for testing),

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

>
> > + 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;