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, 11 insertions, 0 deletions

diff --git a/fs/xfs/libxfs/xfs_metafile.h b/fs/xfs/libxfs/xfs_metafile.h
index acec400123db..8d8f08a6071c 100644
--- a/fs/xfs/libxfs/xfs_metafile.h
+++ b/fs/xfs/libxfs/xfs_metafile.h
@@ -21,6 +21,17 @@ void xfs_metafile_set_iflag(struct xfs_trans *tp, struct xfs_inode *ip,
 		enum xfs_metafile_type metafile_type);
 void xfs_metafile_clear_iflag(struct xfs_trans *tp, struct xfs_inode *ip);
 
+/* Space reservations for metadata inodes. */
+struct xfs_alloc_arg;
+
+bool xfs_metafile_resv_critical(struct xfs_inode *ip);
+void xfs_metafile_resv_alloc_space(struct xfs_inode *ip,
+		struct xfs_alloc_arg *args);
+void xfs_metafile_resv_free_space(struct xfs_inode *ip, struct xfs_trans *tp,
+		xfs_filblks_t len);
+void xfs_metafile_resv_free(struct xfs_inode *ip);
+int xfs_metafile_resv_init(struct xfs_inode *ip, xfs_filblks_t ask);
+
 /* Code specific to kernel/userspace; must be provided externally. */
 
 int xfs_trans_metafile_iget(struct xfs_trans *tp, xfs_ino_t ino,