xfs: compile out v4 support if disabled

Add a few strategic IS_ENABLED statements to let the compiler eliminate
unused code when CONFIG_XFS_SUPPORT_V4 is disabled.

This saves multiple kilobytes of .text in my .config:

$ size xfs.o.*
text	   data	    bss	    dec	    hex	filename
1363633	 294836	    592	1659061	 1950b5	xfs.o.new
1371453	 294868	    592	1666913	 196f61	xfs.o.old
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_mount.h

@@ -332,19 +332,10 @@ static inline void xfs_add_ ## name (struct xfs_mount *mp) \
 __XFS_ADD_FEAT(attr, ATTR)
 __XFS_HAS_FEAT(nlink, NLINK)
 __XFS_ADD_FEAT(quota, QUOTA)
-__XFS_HAS_FEAT(align, ALIGN)
 __XFS_HAS_FEAT(dalign, DALIGN)
-__XFS_HAS_FEAT(logv2, LOGV2)
 __XFS_HAS_FEAT(sector, SECTOR)
-__XFS_HAS_FEAT(extflg, EXTFLG)
 __XFS_HAS_FEAT(asciici, ASCIICI)
-__XFS_HAS_FEAT(lazysbcount, LAZYSBCOUNT)
-__XFS_ADD_FEAT(attr2, ATTR2)
 __XFS_HAS_FEAT(parent, PARENT)
-__XFS_ADD_FEAT(projid32, PROJID32)
-__XFS_HAS_FEAT(crc, CRC)
-__XFS_HAS_FEAT(v3inodes, V3INODES)
-__XFS_HAS_FEAT(pquotino, PQUOTINO)
 __XFS_HAS_FEAT(ftype, FTYPE)
 __XFS_HAS_FEAT(finobt, FINOBT)
 __XFS_HAS_FEAT(rmapbt, RMAPBT)
@@ -358,6 +349,37 @@ __XFS_HAS_FEAT(needsrepair, NEEDSREPAIR)
 __XFS_HAS_FEAT(large_extent_counts, NREXT64)
 __XFS_HAS_FEAT(exchange_range, EXCHANGE_RANGE)
 
+/*
+ * Some features are always on for v5 file systems, allow the compiler to
+ * eliminiate dead code when building without v4 support.
+ */
+#define __XFS_HAS_V4_FEAT(name, NAME) \
+static inline bool xfs_has_ ## name (struct xfs_mount *mp) \
+{ \
+	return !IS_ENABLED(CONFIG_XFS_SUPPORT_V4) || \
+		(mp->m_features & XFS_FEAT_ ## NAME); \
+}
+
+#define __XFS_ADD_V4_FEAT(name, NAME) \
+	__XFS_HAS_V4_FEAT(name, NAME); \
+static inline void xfs_add_ ## name (struct xfs_mount *mp) \
+{ \
+	if (IS_ENABLED(CONFIG_XFS_SUPPORT_V4)) { \
+		mp->m_features |= XFS_FEAT_ ## NAME; \
+		xfs_sb_version_add ## name(&mp->m_sb); \
+	} \
+}
+
+__XFS_HAS_V4_FEAT(align, ALIGN)
+__XFS_HAS_V4_FEAT(logv2, LOGV2)
+__XFS_HAS_V4_FEAT(extflg, EXTFLG)
+__XFS_HAS_V4_FEAT(lazysbcount, LAZYSBCOUNT)
+__XFS_ADD_V4_FEAT(attr2, ATTR2)
+__XFS_ADD_V4_FEAT(projid32, PROJID32)
+__XFS_HAS_V4_FEAT(v3inodes, V3INODES)
+__XFS_HAS_V4_FEAT(crc, CRC)
+__XFS_HAS_V4_FEAT(pquotino, PQUOTINO)
+
 /*
  * Mount features
  *

fs/xfs/xfs_super.c

@@ -1580,17 +1580,21 @@ xfs_fs_fill_super(
 	if (error)
 		goto out_free_sb;
 
-	/* V4 support is undergoing deprecation. */
-	if (!xfs_has_crc(mp)) {
-#ifdef CONFIG_XFS_SUPPORT_V4
+	/*
+	 * V4 support is undergoing deprecation.
+	 *
+	 * Note: this has to use an open coded m_features check as xfs_has_crc
+	 * always returns false for !CONFIG_XFS_SUPPORT_V4.
+	 */
+	if (!(mp->m_features & XFS_FEAT_CRC)) {
+		if (!IS_ENABLED(CONFIG_XFS_SUPPORT_V4)) {
+			xfs_warn(mp,
+	"Deprecated V4 format (crc=0) not supported by kernel.");
+			error = -EINVAL;
+			goto out_free_sb;
+		}
 		xfs_warn_once(mp,
 	"Deprecated V4 format (crc=0) will not be supported after September 2030.");
-#else
-		xfs_warn(mp,
-	"Deprecated V4 format (crc=0) not supported by kernel.");
-		error = -EINVAL;
-		goto out_free_sb;
-#endif
 	}
 
 	/* ASCII case insensitivity is undergoing deprecation. */