xfs: simplify XLOG_SECTOR_ROUND*()

XLOG_SECTOR_ROUNDUP_BBCOUNT() is defined in "fs/xfs/xfs_log_recover.c" in an overly-complicated way. It is basically roundup(), but that is not at all clear from its definition. (Actually, there is another macro round_up() that applies for power-of-two-based masks which I'll be using here.)

The operands in XLOG_SECTOR_ROUNDUP_BBCOUNT() are basically the block number (bbs) and the log sector basic block mask (log->l_sectbb_mask). I'll call them B and M for this discussion.

The macro computes is value this way:

	M && (B & M) ? (B + M + 1) & ~M : B

Put another way, we can break it into 3 cases:

	1)  ! M          -> B			# 0 mask, no effect
	2)  ! (B & M)    -> B			# sector aligned
	3)  M && (B & M) -> (B + M + 1) & ~M	# round up otherwise

The round_up() macro is cleverly defined using a value, v, and a power-of-2, p, and the result is the nearest multiple of p greater than or equal to v. Its value is computed something like this:

	((v - 1) | (p - 1)) + 1

Let's consider using this in the context of the 3 cases above.

When p = 2^0 = 1, the result boils down to ((v - 1) | 0) + 1, so it just translates any value v to itself. That handles case (1) above.

When p = 2^n, n > 0, we know that (p - 1) will be a mask with all n bits 0..n-1 set. The condition in this case occurs when none of those mask bits is set in the value v provided. If that is the case, subtracting 1 from v will have 1's in all those lower bits (at least). Therefore, OR-ing the mask with that decremented value has no effect, so adding the 1 back again will just translate the v to itself. This handles case (2).

Otherwise, the value v is greater than some multiple of p, and decrementing it will produce a result greater than or equal to that multiple. OR-ing in the mask will produce a value 1 less than the next multiple of p, so finally adding 1 back will result in the desired rounded-up value. This handles case (3).

Hopefully this is convincing.

While I was at it, I converted XLOG_SECTOR_ROUNDDOWN_BLKNO() to use the round_down() macro.

---
fs/xfs/xfs_log_recover.c | 9 ++++-----

 fs/xfs/xfs_log_recover.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

Index: b/fs/xfs/xfs_log_recover.c
===================================================================
@@ -61,14 +61,13 @@ STATIC void	xlog_recover_check_summary(x
  * Sector aligned buffer routines for buffer create/read/write/access
  */
 
-#define XLOG_SECTOR_ROUNDUP_BBCOUNT(log, bbs)	\
-	( ((log)->l_sectbb_mask && (bbs & (log)->l_sectbb_mask)) ? \
-	((bbs + (log)->l_sectbb_mask + 1) & ~(log)->l_sectbb_mask) : (bbs) )
-#define XLOG_SECTOR_ROUNDDOWN_BLKNO(log, bno)	((bno) & ~(log)->l_sectbb_mask)
-
 /* Number of basic blocks in a log sector */
 #define xlog_sectbb(log) (1 << (log)->l_sectbb_log)
 
+#define XLOG_SECTOR_ROUNDUP_BBCOUNT(log, bbs) round_up((bbs), xlog_sectbb(log))
+#define XLOG_SECTOR_ROUNDDOWN_BLKNO(log, bno) \
+		round_down((bno), xlog_sectbb(log))
+
 STATIC xfs_buf_t *
 xlog_get_bp(
 	xlog_t		*log,

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