xfs: estimate post-merge refcounts correctly

Source kernel commit: 65ef77b98122a713774bb2f7b90fca9da39c9ea5

Upon enabling fsdax + reflink for XFS, xfs/179 began to report refcount
metadata corruptions after being run.  Specifically, xfs_repair noticed
single-block refcount records that could be combined but had not been.

The root cause of this is improper MAXREFCOUNT edge case handling in
xfs_refcount_merge_extents.  When we're trying to find candidates for a
refcount btree record merge, we compute the refcount attribute of the
merged record, but we fail to account for the fact that once a record
hits rc_refcount == MAXREFCOUNT, it is pinned that way forever.  Hence
the computed refcount is wrong, and we fail to merge the extents.

Fix this by adjusting the merge predicates to compute the adjusted
refcount correctly.

Fixes: 3172725814f9 ("xfs: adjust refcount of an extent of blocks in refcount btree")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

diff --git a/libxfs/xfs_refcount.c b/libxfs/xfs_refcount.c
index f6167c5fc1dfc273d9776db16ee1248de743fc42..29258bddbcd55e63781116b9b9612547633b02ee 100644 (file)
--- a/libxfs/xfs_refcount.c
+++ b/libxfs/xfs_refcount.c
@@ -819,6 +819,17 @@ xfs_refc_valid(
        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,
@@ -830,6 +841,7 @@ xfs_refc_want_merge_center(
        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
@@ -845,9 +857,10 @@ xfs_refc_want_merge_center(
                return false;
 
        /* The shoulder record refcounts must match the new refcount. */
-       if (left->rc_refcount != cleft->rc_refcount + adjust)
+       new_refcount = xfs_refc_merge_refcount(cleft, adjust);
+       if (left->rc_refcount != new_refcount)
                return false;
-       if (right->rc_refcount != cleft->rc_refcount + adjust)
+       if (right->rc_refcount != new_refcount)
                return false;
 
        /*
@@ -870,6 +883,7 @@ xfs_refc_want_merge_left(
        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
@@ -880,7 +894,8 @@ xfs_refc_want_merge_left(
                return false;
 
        /* Left shoulder record refcount must match the new refcount. */
-       if (left->rc_refcount != cleft->rc_refcount + adjust)
+       new_refcount = xfs_refc_merge_refcount(cleft, adjust);
+       if (left->rc_refcount != new_refcount)
                return false;
 
        /*
@@ -902,6 +917,7 @@ xfs_refc_want_merge_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
@@ -912,7 +928,8 @@ xfs_refc_want_merge_right(
                return false;
 
        /* Right shoulder record refcount must match the new refcount. */
-       if (right->rc_refcount != cright->rc_refcount + adjust)
+       new_refcount = xfs_refc_merge_refcount(cright, adjust);
+       if (right->rc_refcount != new_refcount)
                return false;
 
        /*