xfs: don't look for end of extent further than necessary in xfs_rtallocate_extent_near()

As explained in the previous commit, xfs_rtallocate_extent_near() looks
for the end of a free extent when searching backwards from the target
bitmap block. Since the previous commit, it searches from the last
bitmap block it checked to the bitmap block containing the start of the
extent.

This may still be more than necessary, since the free extent may not be
that long. We know the maximum size of the free extent from the realtime
summary. Use that to compute how many bitmap blocks we actually need to
check.
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_rtalloc.c

@@ -527,13 +527,30 @@ xfs_rtallocate_extent_near(
 			 * On the negative side of the starting location.
 			 */
 			else {		/* i < 0 */
+				int	maxblocks;
+
+				/*
+				 * Loop backwards to find the end of the extent
+				 * we found in the realtime summary.
+				 *
+				 * maxblocks is the maximum possible number of
+				 * bitmap blocks from the start of the extent
+				 * to the end of the extent.
+				 */
+				if (maxlog == 0)
+					maxblocks = 0;
+				else if (maxlog < mp->m_blkbit_log)
+					maxblocks = 1;
+				else
+					maxblocks = 2 << (maxlog - mp->m_blkbit_log);
+
 				/*
-				 * Loop backwards through the bitmap blocks
-				 * from where we last checked down to where we
-				 * are now.  There should be an extent which
-				 * ends in this bitmap block and is long
-				 * enough.
+				 * We need to check bbno + i + maxblocks down to
+				 * bbno + i. We already checked bbno down to
+				 * bbno + j + 1, so we don't need to check those
+				 * again.
 				 */
+				j = min(i + maxblocks, j);
 				for (; j >= i; j--) {
 					error = xfs_rtallocate_extent_block(args,
 							bbno + j, minlen,