xfs: sanitise sb_bad_features2 handling

We currently have to ensure that every time we update sb_features2
that we update sb_bad_features2. Now that we log and format the
superblock in it's entirety we actually don't have to care because
we can simply update the sb_bad_features2 when we format it into the
buffer. This removes the need for anything but the mount and
superblock formatting code to care about sb_bad_features2, and
hence removes the possibility that we forget to update bad_features2
when necessary in the future.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

3 files changed, 25 insertions, 20 deletions

diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index fbd6da263571..749c86102794 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -151,10 +151,13 @@ typedef struct xfs_sb {
         __uint32_t      sb_features2;   /* additional feature bits */
 
         /*
-         * bad features2 field as a result of failing to pad the sb
-         * structure to 64 bits. Some machines will be using this field
-         * for features2 bits. Easiest just to mark it bad and not use
-         * it for anything else.
+         * bad features2 field as a result of failing to pad the sb structure to
+         * 64 bits. Some machines will be using this field for features2 bits.
+         * Easiest just to mark it bad and not use it for anything else.
+         *
+         * This is not kept up to date in memory; it is always overwritten by
+         * the value in sb_features2 when formatting the incore superblock to
+         * the disk buffer.
          */
         __uint32_t      sb_bad_features2;
 
@@ -453,13 +456,11 @@ static inline void xfs_sb_version_addattr2(struct xfs_sb *sbp)
 {
         sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
         sbp->sb_features2 |= XFS_SB_VERSION2_ATTR2BIT;
-        sbp->sb_bad_features2 |= XFS_SB_VERSION2_ATTR2BIT;
 }
 
 static inline void xfs_sb_version_removeattr2(struct xfs_sb *sbp)
 {
         sbp->sb_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
-        sbp->sb_bad_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
         if (!sbp->sb_features2)
                 sbp->sb_versionnum &= ~XFS_SB_VERSION_MOREBITSBIT;
 }
@@ -475,7 +476,6 @@ static inline void xfs_sb_version_addprojid32bit(struct xfs_sb *sbp)
 {
         sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
         sbp->sb_features2 |= XFS_SB_VERSION2_PROJID32BIT;
-        sbp->sb_bad_features2 |= XFS_SB_VERSION2_PROJID32BIT;
 }
 
 /*
diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
index 63f814872dfb..b0a5fe95a3e2 100644
--- a/fs/xfs/libxfs/xfs_sb.c
+++ b/fs/xfs/libxfs/xfs_sb.c
@@ -497,7 +497,6 @@ xfs_sb_to_disk(
         to->sb_fdblocks = cpu_to_be64(from->sb_fdblocks);
         to->sb_frextents = cpu_to_be64(from->sb_frextents);
 
-
         to->sb_flags = from->sb_flags;
         to->sb_shared_vn = from->sb_shared_vn;
         to->sb_inoalignmt = cpu_to_be32(from->sb_inoalignmt);
@@ -507,6 +506,13 @@ xfs_sb_to_disk(
         to->sb_logsectlog = from->sb_logsectlog;
         to->sb_logsectsize = cpu_to_be16(from->sb_logsectsize);
         to->sb_logsunit = cpu_to_be32(from->sb_logsunit);
+
+        /*
+         * We need to ensure that bad_features2 always matches features2.
+         * Hence we enforce that here rather than having to remember to do it
+         * everywhere else that updates features2.
+         */
+        from->sb_bad_features2 = from->sb_features2;
         to->sb_features2 = cpu_to_be32(from->sb_features2);
         to->sb_bad_features2 = cpu_to_be32(from->sb_bad_features2);
 
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 5ef9aa2bfa19..4fa80e63eea2 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -640,25 +640,24 @@ xfs_mountfs(
         xfs_sb_mount_common(mp, sbp);
 
         /*
-         * 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.
+         * 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 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. The
+         * superblock writeback code ensures the new sb_features2 is copied to
+         * sb_bad_features2 before it is logged or written to disk.
          */
         if (xfs_sb_has_mismatched_features2(sbp)) {
                 xfs_warn(mp, "correcting sb_features alignment problem");
                 sbp->sb_features2 |= sbp->sb_bad_features2;
-                sbp->sb_bad_features2 = sbp->sb_features2;
                 mp->m_update_sb = true;
 
                 /*