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[PATCH 3/10] xfs: simplify XLOG_SECTOR_ROUND*()

To: XFS Mailing List <xfs@xxxxxxxxxxx>
Subject: [PATCH 3/10] xfs: simplify XLOG_SECTOR_ROUND*()
From: Alex Elder <aelder@xxxxxxx>
Date: Fri, 09 Apr 2010 17:27:27 -0500
Reply-to: aelder@xxxxxxx
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.

Signed-off-by: Alex Elder <aelder@xxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>

---
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
===================================================================
--- a/fs/xfs/xfs_log_recover.c
+++ 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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