[PATCHv2 9/10] xfs: a few more minor xfs_log_recover.c cleanups

[Alex Elder <aelder@xxxxxxx>]

Minor things that didn't seem to warrant their own individual
patches:
- In xlog_bread_noalign(), reorder assertions so the buffer pointer
  is known to be non-null before attempting to dereference it.
- Add a more descriptive header comment for xlog_find_verify_cycle().
- Make a few additions to the comments in xlog_find_head().  Also
  rearrange some expressions in a few spots to produce the same
  result, but in a way that seems more clear what's being computed.

Signed-off-by: Alex Elder <aelder@xxxxxxx>

---
fs/xfs/xfs_log_recover.c |   83 ++++++++++++++++++++++++-----------------------
 fs/xfs/xfs_log_recover.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

Index: b/fs/xfs/xfs_log_recover.c
===================================================================
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -166,8 +166,8 @@ xlog_bread_noalign(
        nbblks = round_up(nbblks, xlog_sectbb(log));
 
        ASSERT(nbblks > 0);
-       ASSERT(BBTOB(nbblks) <= XFS_BUF_SIZE(bp));
        ASSERT(bp);
+       ASSERT(BBTOB(nbblks) <= XFS_BUF_SIZE(bp));
 
        XFS_BUF_SET_ADDR(bp, log->l_logBBstart + blk_no);
        XFS_BUF_READ(bp);
@@ -387,8 +387,11 @@ xlog_find_cycle_start(
  * test.  If the region is completely good, we end up returning the same
  * last block number.
  *
- * Set blkno to -1 if we encounter no errors.  This is an invalid block number
- * since we don't ever expect logs to get this large.
+ * If we encounter a basic block containing stop_on_cycle_no as its
+ * cycle number, return that basic block offset in *new_blk.  If no
+ * basic blocks in the range specified contain stop_on_cycle_no, set
+ * *new_blk to -1.  This is an invalid block number since we don't
+ * ever expect logs to get this large.
  */
 STATIC int
 xlog_find_verify_cycle(
@@ -662,7 +665,7 @@ xlog_find_head(
                 * In this case we want to find the first block with cycle
                 * number matching last_half_cycle.  We expect the log to be
                 * some variation on
-                *        x + 1 ... | x ...
+                *        x + 1 ... | x ... | x
                 * The first block with cycle number x (last_half_cycle) will
                 * be where the new head belongs.  First we do a binary search
                 * for the first occurrence of last_half_cycle.  The binary
@@ -672,11 +675,13 @@ xlog_find_head(
                 * the log, then we look for occurrences of last_half_cycle - 1
                 * at the end of the log.  The cases we're looking for look
                 * like
-                *        x + 1 ... | x | x + 1 | x ...
-                *                               ^ binary search stopped here
+                *                               v binary search stopped here
+                *        x + 1 ... | x | x + 1 | x ... | x
+                *                   ^ but we want to locate this spot
                 * or
-                *        x + 1 ... | x ... | x - 1 | x
                 *        <---------> less than scan distance
+                *        x + 1 ... | x ... | x - 1 | x
+                *                           ^ we want to locate this spot
                 */
                stop_on_cycle = last_half_cycle;
                if ((error = xlog_find_cycle_start(log, bp, first_blk,
@@ -732,9 +737,9 @@ xlog_find_head(
                 * certainly not the head of the log.  By searching for
                 * last_half_cycle-1 we accomplish that.
                 */
-               start_blk = log_bbnum - num_scan_bblks + head_blk;
                ASSERT(head_blk <= INT_MAX &&
-                       (xfs_daddr_t) num_scan_bblks - head_blk >= 0);
+                       (xfs_daddr_t) num_scan_bblks >= head_blk);
+               start_blk = log_bbnum - (num_scan_bblks - head_blk);
                if ((error = xlog_find_verify_cycle(log, start_blk,
                                        num_scan_bblks - (int)head_blk,
                                        (stop_on_cycle - 1), &new_blk)))
@@ -781,7 +786,7 @@ fine_tune:
                if ((error = xlog_find_verify_log_record(log, start_blk,
                                                        &head_blk, 0)) == -1) {
                        /* We hit the beginning of the log during our search */
-                       start_blk = log_bbnum - num_scan_bblks + head_blk;
+                       start_blk = log_bbnum - (num_scan_bblks - head_blk);
                        new_blk = log_bbnum;
                        ASSERT(start_blk <= INT_MAX &&
                                (xfs_daddr_t) log_bbnum-start_blk >= 0);