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xfs: simplify XLOG_SECTOR_ROUND*()

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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@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
This commit is contained in:
Alex Elder 2010-04-13 15:22:40 +10:00
parent 6881a229f6
commit 8511998baa
1 changed files with 4 additions and 5 deletions
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9
fs/xfs/xfs_log_recover.c
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@ -61,14 +61,13 @@ STATIC void xlog_recover_check_summary(xlog_t *);
* 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,