[PATCH 5/7] xfs: Use 2^27 as the maximum number of directory extents

[Chandan Babu R <chandanrlinux@gmail.com>]

The maximum number of extents that can be used by a directory can be
calculated as shown below. (FS block size is assumed to be 512 bytes
since the smallest allowed block size can create a BMBT of maximum
possible height).

Maximum number of extents in data space =
XFS_DIR2_SPACE_SIZE / 2^9 = 32GiB / 2^9 = 2^26.

Maximum number (theoretically) of extents in leaf space =
32GiB / 2^9 = 2^26.

Maximum number of entries in a free space index block
= (512 - (sizeof struct xfs_dir3_free_hdr)) / (sizeof struct
                                               xfs_dir2_data_off_t)
= (512 - 64) / 2 = 224

Maximum number of extents in free space index =
(Maximum number of extents in data segment) / 224 =
2^26 / 224 = ~2^18

Maximum number of extents in a directory =
Maximum number of extents in data space +
Maximum number of extents in leaf space +
Maximum number of extents in free space index =
2^26 + 2^26 + 2^18 = ~2^27

This commit defines the macro MAXDIREXTNUM to have the value 2^27 and
this in turn is used in calculating the maximum height of a directory
BMBT.

Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
---
 fs/xfs/libxfs/xfs_bmap.c  | 2 +-
 fs/xfs/libxfs/xfs_types.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 8b0029b3cecf..f75b70ae7b1f 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -81,7 +81,7 @@ xfs_bmap_compute_maxlevels(
 	if (whichfork == XFS_DATA_FORK) {
 		sz = XFS_BMDR_SPACE_CALC(MINDBTPTRS);
 		if (dir_bmbt)
-			maxleafents = MAXEXTNUM;
+			maxleafents = MAXDIREXTNUM;
 		else
 			maxleafents = MAXEXTNUM;
 	} else {
diff --git a/fs/xfs/libxfs/xfs_types.h b/fs/xfs/libxfs/xfs_types.h
index 397d94775440..0a3041ad5bec 100644
--- a/fs/xfs/libxfs/xfs_types.h
+++ b/fs/xfs/libxfs/xfs_types.h
@@ -60,6 +60,7 @@ typedef void *		xfs_failaddr_t;
  */
 #define	MAXEXTLEN	((xfs_extlen_t)0x001fffff)	/* 21 bits */
 #define	MAXEXTNUM	((xfs_extnum_t)0x7fffffff)	/* signed int */
+#define	MAXDIREXTNUM	((xfs_extnum_t)0x7ffffff)	/* 27 bits */
 #define	MAXAEXTNUM	((xfs_aextnum_t)0x7fff)		/* signed short */
 
 /*
-- 
2.20.1