don't change s_bitmap_maxbytes

snapshot file size is limited by 32bit logical block address
the same as extent mapped file, so there is no need to change
s_bitmap_maxbytes which also affects other indirect mapped files.
Instead, use s_maxbytes to limit snapshot files seek and
don't allow snapshot files size change by truncate syscall.

fs/ext4/file.c

@@ -230,7 +230,12 @@ loff_t ext4_llseek(struct file *file, loff_t offset, int origin)
 	struct inode *inode = file->f_mapping->host;
 	loff_t maxbytes;
 
+#ifdef CONFIG_EXT4_FS_SNAPSHOT_FILE_HUGE
+	if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
+			!ext4_snapshot_file(inode))
+#else
 	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
+#endif
 		maxbytes = EXT4_SB(inode->i_sb)->s_bitmap_maxbytes;
 	else
 		maxbytes = inode->i_sb->s_maxbytes;

fs/ext4/inode.c

@@ -5232,15 +5232,6 @@ void ext4_truncate(struct inode *inode)
 	ext4_lblk_t last_block;
 	unsigned blocksize = inode->i_sb->s_blocksize;
 
-#ifdef CONFIG_EXT4_FS_SNAPSHOT_FILE_HUGE
-	/* prevent partial truncate of snapshot files */
-	if (ext4_snapshot_file(inode) && inode->i_size != 0) {
-		snapshot_debug(1, "snapshot file (%lu) cannot be partly "
-				"truncated!\n", inode->i_ino);
-		return;
-	}
-
-#endif
 #ifdef CONFIG_EXT4_FS_SNAPSHOT_FILE_PERM
 	/* prevent truncate of files on snapshot list */
 	if (ext4_snapshot_list(inode)) {
@@ -6184,6 +6175,14 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
 	}
 
 	if (attr->ia_valid & ATTR_SIZE) {
+#ifdef CONFIG_EXT4_FS_SNAPSHOT_FILE_HUGE
+		/* prevent size modification of snapshot files */
+		if (ext4_snapshot_file(inode) && attr->ia_size != 0) {
+			snapshot_debug(1, "snapshot file (%lu) only can be "
+					"truncated to 0!\n", inode->i_ino);
+			return -EPERM;
+		}
+#endif
 		if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))) {
 			struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 

fs/ext4/super.c

@@ -2484,11 +2484,7 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
 
 	res += 1LL << (bits-2);
 	res += 1LL << (2*(bits-2));
-#ifdef CONFIG_EXT4_FS_SNAPSHOT_FILE_HUGE
-	res += (1LL + EXT4_SNAPSHOT_EXTRA_TIND_BLOCKS) << (3*(bits-2));
-#else
 	res += 1LL << (3*(bits-2));
-#endif
 	res <<= bits;
 	if (res > upper_limit)
 		res = upper_limit;