xfs: automatic freeing of freshly allocated unwritten space

As mentioned in the previous commit, online repair wants to allocate
space to write out a new metadata structure, and it also wants to hedge
against system crashes during repairs by logging (and later cancelling)
EFIs to free the space if we crash before committing the new data
structure.

Therefore, create a trio of functions to schedule automatic reaping of
freshly allocated unwritten space.  xfs_alloc_schedule_autoreap creates
a paused EFI representing the space we just allocated.  Once the
allocations are made and the autoreaps scheduled, we can start writing
to disk.

If the writes succeed, xfs_alloc_cancel_autoreap marks the EFI work
items as stale and unpauses the pending deferred work item.  Assuming
that's done in the same transaction that commits the new structure into
the filesystem, we guarantee that either the new object is fully
visible, or that all the space gets reclaimed.

If the writes succeed but only part of an extent was used, repair must
call the same _cancel_autoreap function to kill the first EFI and then
log a new EFI to free the unused space.  The first EFI is already
committed, so it cannot be changed.

For full extents that aren't used, xfs_alloc_commit_autoreap will
unpause the EFI, which results in the space being freed during the next
_defer_finish cycle.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 5 insertions, 4 deletions

diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
index 3ca23ab8d92a..3e3469504271 100644
--- a/fs/xfs/xfs_extfree_item.c
+++ b/fs/xfs/xfs_extfree_item.c
@@ -453,7 +453,7 @@ xfs_extent_free_finish_item(
 	struct xfs_extent		*extp;
 	uint				next_extent;
 	xfs_agblock_t			agbno;
-	int				error;
+	int				error = 0;
 
 	xefi = container_of(item, struct xfs_extent_free_item, xefi_list);
 	agbno = XFS_FSB_TO_AGBNO(mp, xefi->xefi_startblock);
@@ -473,9 +473,10 @@ xfs_extent_free_finish_item(
 	 * the existing EFI, and so we need to copy all the unprocessed extents
 	 * in this EFI to the EFD so this works correctly.
 	 */
-	error = __xfs_free_extent(tp, xefi->xefi_pag, agbno,
-			xefi->xefi_blockcount, &oinfo, xefi->xefi_agresv,
-			xefi->xefi_flags & XFS_EFI_SKIP_DISCARD);
+	if (!(xefi->xefi_flags & XFS_EFI_CANCELLED))
+		error = __xfs_free_extent(tp, xefi->xefi_pag, agbno,
+				xefi->xefi_blockcount, &oinfo, xefi->xefi_agresv,
+				xefi->xefi_flags & XFS_EFI_SKIP_DISCARD);
 	if (error == -EAGAIN) {
 		xfs_efd_from_efi(efdp);
 		return error;