[RFC PATCH] xfs: use log_incompat feature instead of speculate matching

[Gao Xiang <hsiangkao@aol.com>]

From: Gao Xiang <hsiangkao@redhat.com>

Use a log_incompat feature just to be safe.
If the current mount is in RO state, it will defer
to next RW remount.

Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
---

After some careful thinking, I think it's probably not working for
supported V4 XFS filesystem. So, I think we'd probably insist on the
previous way (correct me if I'm wrong)...

(since xfs_sb_to_disk() refuses to set up any feature bits for non V5
 fses. That is another awkward setting here (doesn't write out/check
 feature bits for V4 even though using V4 sb reserved fields) and
 unless let V4 completely RO since this commit. )

Just send out as a RFC patch. Not fully tested after I thought as above.

 fs/xfs/libxfs/xfs_format.h | 4 +++-
 fs/xfs/xfs_inode.c         | 3 ++-
 fs/xfs/xfs_mount.c         | 9 +++++++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 31b7ece985bb..9f6c2766f6a6 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -479,7 +479,9 @@ xfs_sb_has_incompat_feature(
 	return (sbp->sb_features_incompat & feature) != 0;
 }
 
-#define XFS_SB_FEAT_INCOMPAT_LOG_ALL 0
+#define XFS_SB_FEAT_INCOMPAT_LOG_NEW_UNLINK	(1 << 0)
+#define XFS_SB_FEAT_INCOMPAT_LOG_ALL	\
+		(XFS_SB_FEAT_INCOMPAT_LOG_NEW_UNLINK)
 #define XFS_SB_FEAT_INCOMPAT_LOG_UNKNOWN	~XFS_SB_FEAT_INCOMPAT_LOG_ALL
 static inline bool
 xfs_sb_has_incompat_log_feature(
diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 7ee778bcde06..e8eee1437611 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -1952,7 +1952,8 @@ xfs_iunlink_update_bucket(
 	if (!log || log->l_flags & XLOG_RECOVERY_NEEDED) {
 		ASSERT(cur_agino != NULLAGINO);
 
-		if (be32_to_cpu(agi->agi_unlinked[0]) != cur_agino)
+		if (!(mp->m_sb.sb_features_log_incompat &
+		      XFS_SB_FEAT_INCOMPAT_LOG_NEW_UNLINK))
 			bucket_index = cur_agino % XFS_AGI_UNLINKED_BUCKETS;
 	}
 
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index f28c969af272..91d8b22524c6 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -836,6 +836,15 @@ xfs_mountfs(
 		goto out_fail_wait;
 	}
 
+	if (!(sbp->sb_features_log_incompat &
+	      XFS_SB_FEAT_INCOMPAT_LOG_NEW_UNLINK) &&
+	    !(mp->m_flags & XFS_MOUNT_RDONLY)) {
+		xfs_warn(mp, "will switch to long iunlinked list on r/w");
+		sbp->sb_features_log_incompat |=
+				XFS_SB_FEAT_INCOMPAT_LOG_NEW_UNLINK;
+		mp->m_update_sb = true;
+	}
+
 	/* Make sure the summary counts are ok. */
 	error = xfs_check_summary_counts(mp);
 	if (error)
-- 
2.24.0