ext4: optimize Hurd tests when reading/writing inodes

Set a in-memory superblock flag to indicate whether the file system is
designed to support the Hurd.

Also, add a sanity check to make sure the 64-bit feature is not set
for Hurd file systems, since i_file_acl_high conflicts with a
Hurd-specific field.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1001,6 +1001,8 @@ struct ext4_inode_info {
 #define EXT4_MOUNT2_STD_GROUP_SIZE	0x00000002 /* We have standard group
 						      size of blocksize * 8
 						      blocks */
+#define EXT4_MOUNT2_HURD_COMPAT		0x00000004 /* Support HURD-castrated
+						      file systems */
 
 #define clear_opt(sb, opt)		EXT4_SB(sb)->s_mount_opt &= \
 						~EXT4_MOUNT_##opt

fs/ext4/inode.c

@@ -4168,8 +4168,7 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	EXT4_INODE_GET_XTIME(i_atime, inode, raw_inode);
 	EXT4_EINODE_GET_XTIME(i_crtime, ei, raw_inode);
 
-	if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
-	    cpu_to_le32(EXT4_OS_HURD)) {
+	if (likely(!test_opt2(inode->i_sb, HURD_COMPAT))) {
 		inode->i_version = le32_to_cpu(raw_inode->i_disk_version);
 		if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE) {
 			if (EXT4_FITS_IN_INODE(raw_inode, ei, i_version_hi))
@@ -4345,8 +4344,7 @@ static int ext4_do_update_inode(handle_t *handle,
 		goto out_brelse;
 	raw_inode->i_dtime = cpu_to_le32(ei->i_dtime);
 	raw_inode->i_flags = cpu_to_le32(ei->i_flags & 0xFFFFFFFF);
-	if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
-	    cpu_to_le32(EXT4_OS_HURD))
+	if (likely(!test_opt2(inode->i_sb, HURD_COMPAT)))
 		raw_inode->i_file_acl_high =
 			cpu_to_le16(ei->i_file_acl >> 32);
 	raw_inode->i_file_acl_lo = cpu_to_le32(ei->i_file_acl);
@@ -4391,8 +4389,7 @@ static int ext4_do_update_inode(handle_t *handle,
 			raw_inode->i_block[block] = ei->i_data[block];
 	}
 
-	if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
-	    cpu_to_le32(EXT4_OS_HURD)) {
+	if (likely(!test_opt2(inode->i_sb, HURD_COMPAT))) {
 		raw_inode->i_disk_version = cpu_to_le32(inode->i_version);
 		if (ei->i_extra_isize) {
 			if (EXT4_FITS_IN_INODE(raw_inode, ei, i_version_hi))

fs/ext4/super.c

@@ -3580,6 +3580,16 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		       "feature flags set on rev 0 fs, "
 		       "running e2fsck is recommended");
 
+	if (es->s_creator_os == cpu_to_le32(EXT4_OS_HURD)) {
+		set_opt2(sb, HURD_COMPAT);
+		if (EXT4_HAS_INCOMPAT_FEATURE(sb,
+					      EXT4_FEATURE_INCOMPAT_64BIT)) {
+			ext4_msg(sb, KERN_ERR,
+				 "The Hurd can't support 64-bit file systems");
+			goto failed_mount;
+		}
+	}
+
 	if (IS_EXT2_SB(sb)) {
 		if (ext2_feature_set_ok(sb))
 			ext4_msg(sb, KERN_INFO, "mounting ext2 file system "