[XFS] Ensure "both" features2 slots are consistent

Since older kernels may look in the sb_bad_features2 slot for flags,
rather than zeroing it out on fixup, we should make it equal to the
sb_features2 value.

Also, if the ATTR2 flag was not found prior to features2 fixup, it was not
set in the mount flags, so re-check after the fixup so that the current
session will use the feature.

Also fix up the comments to reflect these changes.

SGI-PV: 980085
SGI-Modid: xfs-linux-melb:xfs-kern:30778a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

2 files changed, 29 insertions, 15 deletions

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 4e93c02faf24..8ed164eb9544 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -972,23 +972,35 @@ xfs_mountfs(
         xfs_mount_common(mp, sbp);
 
         /*
-         * Check for a bad features2 field alignment. This happened on
-         * some platforms due to xfs_sb_t not being 64bit size aligned
-         * when sb_features was added and hence the compiler put it in
-         * the wrong place.
+         * 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.
          *
-         * If we detect a bad field, we or the set bits into the existing
-         * features2 field in case it has already been modified and we
-         * don't want to lose any features. Zero the bad one and mark
-         * the two fields as needing updates once the transaction subsystem
-         * is online.
+         * 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_bad_features2(sbp)) {
+        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 = 0;
+                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;
+
         }
 
         /*
@@ -1896,7 +1908,8 @@ xfs_uuid_unmount(
 
 /*
  * Used to log changes to the superblock unit and width fields which could
- * be altered by the mount options. Only the first superblock is updated.
+ * be altered by the mount options, as well as any potential sb_features2
+ * fixup. Only the first superblock is updated.
  */
 STATIC void
 xfs_mount_log_sb(
diff --git a/fs/xfs/xfs_sb.h b/fs/xfs/xfs_sb.h
index b1a83f8ec044..d904efe7f871 100644
--- a/fs/xfs/xfs_sb.h
+++ b/fs/xfs/xfs_sb.h
@@ -320,11 +320,12 @@ static inline int xfs_sb_good_version(xfs_sb_t *sbp)
 #endif /* __KERNEL__ */
 
 /*
- * Detect a bad features2 field
+ * Detect a mismatched features2 field.  Older kernels read/wrote
+ * this into the wrong slot, so to be safe we keep them in sync.
  */
-static inline int xfs_sb_has_bad_features2(xfs_sb_t *sbp)
+static inline int xfs_sb_has_mismatched_features2(xfs_sb_t *sbp)
 {
-        return (sbp->sb_bad_features2 != 0);
+        return (sbp->sb_bad_features2 != sbp->sb_features2);
 }
 
 static inline unsigned xfs_sb_version_tonew(unsigned v)