xfs: allow inode-based btrees to reserve space in the data device

Create a new space reservation scheme so that btree metadata for the
realtime volume can reserve space in the data device to avoid space
underruns.

Back when we were testing the rmap and refcount btrees for the data
device, people observed occasional shutdowns when xfs_btree_split was
called for either of those two btrees.  This happened when certain
operations (mostly writeback ioends) created new rmap or refcount
records, which would expand the size of the btree.  If there were no
free blocks available the allocation would fail and the split would shut
down the filesystem.

I considered pre-reserving blocks for btree expansion at the time of a
write() call, but there wasn't any good way to attach the reservations
to an inode and keep them there all the way to ioend processing.  Unlike
delalloc reservations which have that indlen mechanism, there's no way
to do that for mapped extents; and indlen blocks are given back during
the delalloc -> unwritten transition.

The solution was to reserve sufficient blocks for rmap/refcount btree
expansion at mount time.  This is what the XFS_AG_RESV_* flags provide;
any expansion of those two btrees can come from the pre-reserved space.

This patch brings that pre-reservation ability to inode-rooted btrees so
that the rt rmap and refcount btrees can also save room for future
expansion.

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

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/xfs/xfs_rtalloc.h b/fs/xfs/xfs_rtalloc.h
index 8e2a07b8174b..d87523e6a550 100644
--- a/fs/xfs/xfs_rtalloc.h
+++ b/fs/xfs/xfs_rtalloc.h
@@ -34,6 +34,9 @@ int					/* error */
 xfs_rtmount_inodes(
 	struct xfs_mount	*mp);	/* file system mount structure */
 
+void xfs_rt_resv_free(struct xfs_mount *mp);
+int xfs_rt_resv_init(struct xfs_mount *mp);
+
 /*
  * Grow the realtime area of the filesystem.
  */
@@ -60,6 +63,8 @@ xfs_rtmount_init(
 }
 # define xfs_rtmount_inodes(m)  (((mp)->m_sb.sb_rblocks == 0)? 0 : (-ENOSYS))
 # define xfs_rtunmount_inodes(m)
+# define xfs_rt_resv_free(mp)				((void)0)
+# define xfs_rt_resv_init(mp)				(0)
 #endif	/* CONFIG_XFS_RT */
 
 #endif	/* __XFS_RTALLOC_H__ */