xfs: make sure maxlen is still congruent with prod when rounding down

In commit 2a6ca4ba, we tried to fix an overflow problem in the
realtime allocator that was caused by an overly large maxlen value
causing xfs_rtcheck_range to run off the end of the realtime bitmap.
Unfortunately, there is a subtle bug here -- maxlen (and minlen) both
have to be aligned with @prod, but @prod can be larger than 1 if the
user has set an extent size hint on the file, and that extent size hint
is larger than the realtime extent size.

If the rt free space extents are not aligned to this file's extszhint
because other files without extent size hints allocated space (or the
number of rt extents is similarly not aligned), then it's possible that
maxlen after clamping to sb_rextents will no longer be aligned to prod.
The allocation will succeed just fine, but we still trip the assertion.

Fix the problem by reducing maxlen by any misalignment with prod.  While
we're at it, split the assertions into two so that we can tell which
value had the bad alignment.

Fixes: 2a6ca4ba ("xfs: make sure the rt allocator doesn't run off the end")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_rtalloc.c

@@ -211,6 +211,23 @@ xfs_rtallocate_range(
 	return error;
 }
 
+/*
+ * Make sure we don't run off the end of the rt volume.  Be careful that
+ * adjusting maxlen downwards doesn't cause us to fail the alignment checks.
+ */
+static inline xfs_extlen_t
+xfs_rtallocate_clamp_len(
+	struct xfs_mount	*mp,
+	xfs_rtblock_t		startrtx,
+	xfs_extlen_t		rtxlen,
+	xfs_extlen_t		prod)
+{
+	xfs_extlen_t		ret;
+
+	ret = min(mp->m_sb.sb_rextents, startrtx + rtxlen) - startrtx;
+	return rounddown(ret, prod);
+}
+
 /*
  * Attempt to allocate an extent minlen<=len<=maxlen starting from
  * bitmap block bbno.  If we don't get maxlen then use prod to trim
@@ -248,7 +265,7 @@ xfs_rtallocate_extent_block(
 	     i <= end;
 	     i++) {
 		/* Make sure we don't scan off the end of the rt volume. */
-		maxlen = min(mp->m_sb.sb_rextents, i + maxlen) - i;
+		maxlen = xfs_rtallocate_clamp_len(mp, i, maxlen, prod);
 
 		/*
 		 * See if there's a free extent of maxlen starting at i.
@@ -355,7 +372,8 @@ xfs_rtallocate_extent_exact(
 	int		isfree;		/* extent is free */
 	xfs_rtblock_t	next;		/* next block to try (dummy) */
 
-	ASSERT(minlen % prod == 0 && maxlen % prod == 0);
+	ASSERT(minlen % prod == 0);
+	ASSERT(maxlen % prod == 0);
 	/*
 	 * Check if the range in question (for maxlen) is free.
 	 */
@@ -438,7 +456,9 @@ xfs_rtallocate_extent_near(
 	xfs_rtblock_t	n;		/* next block to try */
 	xfs_rtblock_t	r;		/* result block */
 
-	ASSERT(minlen % prod == 0 && maxlen % prod == 0);
+	ASSERT(minlen % prod == 0);
+	ASSERT(maxlen % prod == 0);
+
 	/*
 	 * If the block number given is off the end, silently set it to
 	 * the last block.
@@ -447,7 +467,7 @@ xfs_rtallocate_extent_near(
 		bno = mp->m_sb.sb_rextents - 1;
 
 	/* Make sure we don't run off the end of the rt volume. */
-	maxlen = min(mp->m_sb.sb_rextents, bno + maxlen) - bno;
+	maxlen = xfs_rtallocate_clamp_len(mp, bno, maxlen, prod);
 	if (maxlen < minlen) {
 		*rtblock = NULLRTBLOCK;
 		return 0;
@@ -638,7 +658,8 @@ xfs_rtallocate_extent_size(
 	xfs_rtblock_t	r;		/* result block number */
 	xfs_suminfo_t	sum;		/* summary information for extents */
 
-	ASSERT(minlen % prod == 0 && maxlen % prod == 0);
+	ASSERT(minlen % prod == 0);
+	ASSERT(maxlen % prod == 0);
 	ASSERT(maxlen != 0);
 
 	/*