xfs: fix broken MAXREFCOUNT handling

This series fixes a bug in the refcount code where we don't merge
 records correctly if the refcount is hovering around MAXREFCOUNT.  This
 fixes regressions in xfs/179 when fsdax is enabled.  xfs/179 itself will
 be modified to exploit the bug through the pagecache path.
 
 Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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Merge tag 'maxrefcount-fixes-6.2_2022-12-01' of git://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux into xfs-6.2-mergeD

xfs: fix broken MAXREFCOUNT handling

This series fixes a bug in the refcount code where we don't merge
records correctly if the refcount is hovering around MAXREFCOUNT.  This
fixes regressions in xfs/179 when fsdax is enabled.  xfs/179 itself will
be modified to exploit the bug through the pagecache path.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

* tag 'maxrefcount-fixes-6.2_2022-12-01' of git://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux:
  xfs: estimate post-merge refcounts correctly
  xfs: hoist refcount record merge predicates

fs/xfs/libxfs/xfs_refcount.c

@@ -815,11 +815,136 @@ out_error:
 /* Is this extent valid? */
 static inline bool
 xfs_refc_valid(
-	struct xfs_refcount_irec	*rc)
+	const struct xfs_refcount_irec	*rc)
 {
 	return rc->rc_startblock != NULLAGBLOCK;
 }
 
+static inline xfs_nlink_t
+xfs_refc_merge_refcount(
+	const struct xfs_refcount_irec	*irec,
+	enum xfs_refc_adjust_op		adjust)
+{
+	/* Once a record hits MAXREFCOUNT, it is pinned there forever */
+	if (irec->rc_refcount == MAXREFCOUNT)
+		return MAXREFCOUNT;
+	return irec->rc_refcount + adjust;
+}
+
+static inline bool
+xfs_refc_want_merge_center(
+	const struct xfs_refcount_irec	*left,
+	const struct xfs_refcount_irec	*cleft,
+	const struct xfs_refcount_irec	*cright,
+	const struct xfs_refcount_irec	*right,
+	bool				cleft_is_cright,
+	enum xfs_refc_adjust_op		adjust,
+	unsigned long long		*ulenp)
+{
+	unsigned long long		ulen = left->rc_blockcount;
+	xfs_nlink_t			new_refcount;
+
+	/*
+	 * To merge with a center record, both shoulder records must be
+	 * adjacent to the record we want to adjust.  This is only true if
+	 * find_left and find_right made all four records valid.
+	 */
+	if (!xfs_refc_valid(left)  || !xfs_refc_valid(right) ||
+	    !xfs_refc_valid(cleft) || !xfs_refc_valid(cright))
+		return false;
+
+	/* There must only be one record for the entire range. */
+	if (!cleft_is_cright)
+		return false;
+
+	/* The shoulder record refcounts must match the new refcount. */
+	new_refcount = xfs_refc_merge_refcount(cleft, adjust);
+	if (left->rc_refcount != new_refcount)
+		return false;
+	if (right->rc_refcount != new_refcount)
+		return false;
+
+	/*
+	 * The new record cannot exceed the max length.  ulen is a ULL as the
+	 * individual record block counts can be up to (u32 - 1) in length
+	 * hence we need to catch u32 addition overflows here.
+	 */
+	ulen += cleft->rc_blockcount + right->rc_blockcount;
+	if (ulen >= MAXREFCEXTLEN)
+		return false;
+
+	*ulenp = ulen;
+	return true;
+}
+
+static inline bool
+xfs_refc_want_merge_left(
+	const struct xfs_refcount_irec	*left,
+	const struct xfs_refcount_irec	*cleft,
+	enum xfs_refc_adjust_op		adjust)
+{
+	unsigned long long		ulen = left->rc_blockcount;
+	xfs_nlink_t			new_refcount;
+
+	/*
+	 * For a left merge, the left shoulder record must be adjacent to the
+	 * start of the range.  If this is true, find_left made left and cleft
+	 * contain valid contents.
+	 */
+	if (!xfs_refc_valid(left) || !xfs_refc_valid(cleft))
+		return false;
+
+	/* Left shoulder record refcount must match the new refcount. */
+	new_refcount = xfs_refc_merge_refcount(cleft, adjust);
+	if (left->rc_refcount != new_refcount)
+		return false;
+
+	/*
+	 * The new record cannot exceed the max length.  ulen is a ULL as the
+	 * individual record block counts can be up to (u32 - 1) in length
+	 * hence we need to catch u32 addition overflows here.
+	 */
+	ulen += cleft->rc_blockcount;
+	if (ulen >= MAXREFCEXTLEN)
+		return false;
+
+	return true;
+}
+
+static inline bool
+xfs_refc_want_merge_right(
+	const struct xfs_refcount_irec	*cright,
+	const struct xfs_refcount_irec	*right,
+	enum xfs_refc_adjust_op		adjust)
+{
+	unsigned long long		ulen = right->rc_blockcount;
+	xfs_nlink_t			new_refcount;
+
+	/*
+	 * For a right merge, the right shoulder record must be adjacent to the
+	 * end of the range.  If this is true, find_right made cright and right
+	 * contain valid contents.
+	 */
+	if (!xfs_refc_valid(right) || !xfs_refc_valid(cright))
+		return false;
+
+	/* Right shoulder record refcount must match the new refcount. */
+	new_refcount = xfs_refc_merge_refcount(cright, adjust);
+	if (right->rc_refcount != new_refcount)
+		return false;
+
+	/*
+	 * The new record cannot exceed the max length.  ulen is a ULL as the
+	 * individual record block counts can be up to (u32 - 1) in length
+	 * hence we need to catch u32 addition overflows here.
+	 */
+	ulen += cright->rc_blockcount;
+	if (ulen >= MAXREFCEXTLEN)
+		return false;
+
+	return true;
+}
+
 /*
  * Try to merge with any extents on the boundaries of the adjustment range.
  */
@@ -861,23 +986,15 @@ xfs_refcount_merge_extents(
 		 (cleft.rc_blockcount == cright.rc_blockcount);
 
 	/* Try to merge left, cleft, and right.  cleft must == cright. */
-	ulen = (unsigned long long)left.rc_blockcount + cleft.rc_blockcount +
+	if (xfs_refc_want_merge_center(&left, &cleft, &cright, &right, cequal,
-			right.rc_blockcount;
+				adjust, &ulen)) {
-	if (xfs_refc_valid(&left) && xfs_refc_valid(&right) &&
-	    xfs_refc_valid(&cleft) && xfs_refc_valid(&cright) && cequal &&
-	    left.rc_refcount == cleft.rc_refcount + adjust &&
-	    right.rc_refcount == cleft.rc_refcount + adjust &&
-	    ulen < MAXREFCEXTLEN) {
 		*shape_changed = true;
 		return xfs_refcount_merge_center_extents(cur, &left, &cleft,
 				&right, ulen, aglen);
 	}
 
 	/* Try to merge left and cleft. */
-	ulen = (unsigned long long)left.rc_blockcount + cleft.rc_blockcount;
+	if (xfs_refc_want_merge_left(&left, &cleft, adjust)) {
-	if (xfs_refc_valid(&left) && xfs_refc_valid(&cleft) &&
-	    left.rc_refcount == cleft.rc_refcount + adjust &&
-	    ulen < MAXREFCEXTLEN) {
 		*shape_changed = true;
 		error = xfs_refcount_merge_left_extent(cur, &left, &cleft,
 				agbno, aglen);
@@ -893,10 +1010,7 @@ xfs_refcount_merge_extents(
 	}
 
 	/* Try to merge cright and right. */
-	ulen = (unsigned long long)right.rc_blockcount + cright.rc_blockcount;
+	if (xfs_refc_want_merge_right(&cright, &right, adjust)) {
-	if (xfs_refc_valid(&right) && xfs_refc_valid(&cright) &&
-	    right.rc_refcount == cright.rc_refcount + adjust &&
-	    ulen < MAXREFCEXTLEN) {
 		*shape_changed = true;
 		return xfs_refcount_merge_right_extent(cur, &right, &cright,
 				aglen);