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

[Gao Xiang <hsiangkao@redhat.com>]

Add a log_incompat (v5) or sb_features2 (v4) feature
of a single long iunlinked list just to be safe. Hence,
older kernels will refuse to replay log for v5 images
or mount entirely for v4 images.

If the current mount is in RO state, it will defer
to the next RW (re)mount to add such flag instead.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
---
Different combinations have been tested (v4/v5 and before/after patch).

Based on the top of
`[PATCH 13/13] xfs: reorder iunlink remove operation in xfs_ifree`
https://lore.kernel.org/r/20200812092556.2567285-14-david@fromorbit.com

Either folding or rearranging this patch would be okay.

Maybe xfsprogs could be also patched as well to change the default
feature setting, but let me send out this first...

(It's possible that I'm still missing something...
 Kindly point out any time.)

 fs/xfs/libxfs/xfs_format.h | 29 +++++++++++++++++++++++++++--
 fs/xfs/xfs_inode.c         |  2 +-
 fs/xfs/xfs_mount.c         |  6 ++++++
 3 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 31b7ece985bb..a859fe601f6e 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -79,12 +79,14 @@ struct xfs_ifork;
 #define XFS_SB_VERSION2_PROJID32BIT	0x00000080	/* 32 bit project id */
 #define XFS_SB_VERSION2_CRCBIT		0x00000100	/* metadata CRCs */
 #define XFS_SB_VERSION2_FTYPE		0x00000200	/* inode type in dir */
+#define XFS_SB_VERSION2_NEW_IUNLINK	0x00000400	/* (v4) new iunlink */
 
 #define	XFS_SB_VERSION2_OKBITS		\
 	(XFS_SB_VERSION2_LAZYSBCOUNTBIT	| \
 	 XFS_SB_VERSION2_ATTR2BIT	| \
 	 XFS_SB_VERSION2_PROJID32BIT	| \
-	 XFS_SB_VERSION2_FTYPE)
+	 XFS_SB_VERSION2_FTYPE		| \
+	 XFS_SB_VERSION2_NEW_IUNLINK)
 
 /* Maximum size of the xfs filesystem label, no terminating NULL */
 #define XFSLABEL_MAX			12
@@ -479,7 +481,9 @@ 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_IUNLINK	(1 << 0)
+#define XFS_SB_FEAT_INCOMPAT_LOG_ALL	\
+		(XFS_SB_FEAT_INCOMPAT_LOG_NEW_IUNLINK)
 #define XFS_SB_FEAT_INCOMPAT_LOG_UNKNOWN	~XFS_SB_FEAT_INCOMPAT_LOG_ALL
 static inline bool
 xfs_sb_has_incompat_log_feature(
@@ -563,6 +567,27 @@ static inline bool xfs_sb_version_hasreflink(struct xfs_sb *sbp)
 		(sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_REFLINK);
 }
 
+static inline bool xfs_sb_has_new_iunlink(struct xfs_sb *sbp)
+{
+	if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5)
+		return sbp->sb_features_log_incompat &
+			XFS_SB_FEAT_INCOMPAT_LOG_NEW_IUNLINK;
+
+	return xfs_sb_version_hasmorebits(sbp) &&
+		(sbp->sb_features2 & XFS_SB_VERSION2_NEW_IUNLINK);
+}
+
+static inline void xfs_sb_add_new_iunlink(struct xfs_sb *sbp)
+{
+	if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) {
+		sbp->sb_features_log_incompat |=
+			XFS_SB_FEAT_INCOMPAT_LOG_NEW_IUNLINK;
+		return;
+	}
+	sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
+	sbp->sb_features2 |= XFS_SB_VERSION2_NEW_IUNLINK;
+}
+
 /*
  * end of superblock version macros
  */
diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 7ee778bcde06..1656ed7dcadf 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -1952,7 +1952,7 @@ 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 (!xfs_sb_has_new_iunlink(&mp->m_sb))
 			bucket_index = cur_agino % XFS_AGI_UNLINKED_BUCKETS;
 	}
 
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index f28c969af272..a3b2e3c3d32f 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -836,6 +836,12 @@ xfs_mountfs(
 		goto out_fail_wait;
 	}
 
+	if (!xfs_sb_has_new_iunlink(sbp)) {
+		xfs_warn(mp, "will switch to long iunlinked list on r/w");
+		xfs_sb_add_new_iunlink(sbp);
+		mp->m_update_sb = true;
+	}
+
 	/* Make sure the summary counts are ok. */
 	error = xfs_check_summary_counts(mp);
 	if (error)
-- 
2.18.1