xfs: invert the realtime summary cache

In commit 355e3532132b ("xfs: cache minimum realtime summary level"), I
added a cache of the minimum level of the realtime summary that has any
free extents. However, it turns out that the _maximum_ level is more
useful for upcoming optimizations, and basically equivalent for the
existing usage. So, let's change the meaning of the cache to be the
maximum level + 1, or 0 if there are no free extents.

For example, if the cache contains:

{0, 4}

then there are no free extents starting in realtime bitmap block 0, and
there are no free extents larger than or equal to 2^4 blocks starting in
realtime bitmap block 1. The cache is a loose upper bound, so there may
or may not be free extents smaller than 2^4 blocks in realtime bitmap
block 1.

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

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index d8769dc5f6dd..9cd1d570d24d 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -101,9 +101,9 @@ typedef struct xfs_mount {
 
 	/*
 	 * Optional cache of rt summary level per bitmap block with the
-	 * invariant that m_rsum_cache[bbno] <= the minimum i for which
-	 * rsum[i][bbno] != 0. Reads and writes are serialized by the rsumip
-	 * inode lock.
+	 * invariant that m_rsum_cache[bbno] > the maximum i for which
+	 * rsum[i][bbno] != 0, or 0 if rsum[i][bbno] == 0 for all i.
+	 * Reads and writes are serialized by the rsumip inode lock.
 	 */
 	uint8_t			*m_rsum_cache;
 	struct xfs_mru_cache	*m_filestream;  /* per-mount filestream data */