+ /*
+ * Check for a mismatched features2 values. Older kernels
+ * read & wrote into the wrong sb offset for sb_features2
+ * on some platforms due to xfs_sb_t not being 64bit size aligned
+ * when sb_features2 was added, which made older superblock
+ * reading/writing routines swap it as a 64-bit value.
+ *
+ * For backwards compatibility, we make both slots equal.
+ *
+ * If we detect a mismatched field, we OR the set bits into the
+ * existing features2 field in case it has already been modified; we
+ * don't want to lose any features. We then update the bad location
+ * with the ORed value so that older kernels will see any features2
+ * flags, and mark the two fields as needing updates once the
+ * transaction subsystem is online.
+ */
+ if (xfs_sb_has_mismatched_features2(sbp)) {
+ cmn_err(CE_WARN,
+ "XFS: correcting sb_features alignment problem");
+ sbp->sb_features2 |= sbp->sb_bad_features2;
+ sbp->sb_bad_features2 = sbp->sb_features2;
+ update_flags |= XFS_SB_FEATURES2 | XFS_SB_BAD_FEATURES2;
+
+ /*
+ * Re-check for ATTR2 in case it was found in bad_features2
+ * slot.
+ */
+ if (xfs_sb_version_hasattr2(&mp->m_sb))
+ mp->m_flags |= XFS_MOUNT_ATTR2;
+
+ }
+