xfs: fix high key handling in the rt allocator's query_range function

Fix some off-by-one errors in xfs_rtalloc_query_range.  The highest key
in the realtime bitmap is always one less than the number of rt extents,
which means that the key clamp at the start of the function is wrong.
The 4th argument to xfs_rtfind_forw is the highest rt extent that we
want to probe, which means that passing 1 less than the high key is
wrong.  Finally, drop the rem variable that controls the loop because we
can compare the iteration point (rtstart) against the high key directly.

The sordid history of this function is that the original commit (fb3c3)
incorrectly passed (high_rec->ar_startblock - 1) as the 'limit' parameter
to xfs_rtfind_forw.  This was wrong because the "high key" is supposed
to be the largest key for which the caller wants result rows, not the
key for the first row that could possibly be outside the range that the
caller wants to see.

A subsequent attempt (8ad56) to strengthen the parameter checking added
incorrect clamping of the parameters to the number of rt blocks in the
system (despite the bitmap functions all taking units of rt extents) to
avoid querying ranges past the end of rt bitmap file but failed to fix
the incorrect _rtfind_forw parameter.  The original _rtfind_forw
parameter error then survived the conversion of the startblock and
blockcount fields to rt extents (a0e5c), and the most recent off-by-one
fix (a3a37) thought it was patching a problem when the end of the rt
volume is not in use, but none of these fixes actually solved the
original problem that the author was confused about the "limit" argument
to xfs_rtfind_forw.

Sadly, all four of these patches were written by this author and even
his own usage of this function and rt testing were inadequate to get
this fixed quickly.

Original-problem: fb3c3de2 ("xfs: add a couple of queries to iterate free extents in the rtbitmap")
Not-fixed-by: 8ad560d2 ("xfs: strengthen rtalloc query range checks")
Not-fixed-by: a0e5c435 ("xfs: fix xfs_rtalloc_rec units")
Fixes: a3a374bf ("xfs: fix off-by-one error in xfs_rtalloc_query_range")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

fs/xfs/libxfs/xfs_rtbitmap.c

@@ -1018,7 +1018,6 @@ xfs_rtalloc_query_range(
 	struct xfs_mount		*mp = tp->t_mountp;
 	xfs_rtblock_t			rtstart;
 	xfs_rtblock_t			rtend;
-	xfs_rtblock_t			rem;
 	int				is_free;
 	int				error = 0;
 
@@ -1027,13 +1026,12 @@ xfs_rtalloc_query_range(
 	if (low_rec->ar_startext >= mp->m_sb.sb_rextents ||
 	    low_rec->ar_startext == high_rec->ar_startext)
 		return 0;
-	if (high_rec->ar_startext > mp->m_sb.sb_rextents)
-		high_rec->ar_startext = mp->m_sb.sb_rextents;
+	high_rec->ar_startext = min(high_rec->ar_startext,
+			mp->m_sb.sb_rextents - 1);
 
 	/* Iterate the bitmap, looking for discrepancies. */
 	rtstart = low_rec->ar_startext;
-	rem = high_rec->ar_startext - rtstart;
-	while (rem) {
+	while (rtstart <= high_rec->ar_startext) {
 		/* Is the first block free? */
 		error = xfs_rtcheck_range(mp, tp, rtstart, 1, 1, &rtend,
 				&is_free);
@@ -1042,7 +1040,7 @@ xfs_rtalloc_query_range(
 
 		/* How long does the extent go for? */
 		error = xfs_rtfind_forw(mp, tp, rtstart,
-				high_rec->ar_startext - 1, &rtend);
+				high_rec->ar_startext, &rtend);
 		if (error)
 			break;
 
@@ -1055,7 +1053,6 @@ xfs_rtalloc_query_range(
 				break;
 		}
 
-		rem -= rtend - rtstart + 1;
 		rtstart = rtend + 1;
 	}