[PATCH] fs: ext4: inode->i_generation not assigned 0.

[Kyungchan Koh <kkc6196@fb.com>]

In fs/ext4/super.c, the function ext4_nfs_get_inode takes as input
"generation" that can be used to specify the generation of the inode to
be returned. When 0 is given as input, then inodes of any generation can
be returned. Therefore, generation 0 is a special case that should be
avoided when assigning generation to inodes.

A new inline function, ext4_inode_set_gen, will take care of the
problem.  Now, inodes cannot have a generation of 0, so this patch fixes
the issue.

Signed-off-by: Kyungchan Koh <kkc6196@fb.com>
---
 fs/ext4/ext4.h   | 8 ++++++++
 fs/ext4/ialloc.c | 2 +-
 fs/ext4/ioctl.c  | 4 ++--
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 3219154..74c6677 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1549,6 +1549,14 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
 		 ino <= le32_to_cpu(EXT4_SB(sb)->s_es->s_inodes_count));
 }
 
+static inline void ext4_inode_set_gen(struct inode *inode,
+				      struct ext4_sb_info *sbi)
+{
+	inode->i_generation = sbi->s_next_generation++;
+	if (!inode->i_generation)
+		inode->i_generation = sbi->s_next_generation++;
+}
+
 /*
  * Inode dynamic state flags
  */
diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index 98ac2f1..d33f6f0 100644
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -1072,7 +1072,7 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
 		goto out;
 	}
 	spin_lock(&sbi->s_next_gen_lock);
-	inode->i_generation = sbi->s_next_generation++;
+	ext4_inode_set_gen(inode, sbi);
 	spin_unlock(&sbi->s_next_gen_lock);
 
 	/* Precompute checksum seed for inode metadata */
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 0c21e22..d52a467 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -160,8 +160,8 @@ static long swap_inode_boot_loader(struct super_block *sb,
 	inode->i_ctime = inode_bl->i_ctime = current_time(inode);
 
 	spin_lock(&sbi->s_next_gen_lock);
-	inode->i_generation = sbi->s_next_generation++;
-	inode_bl->i_generation = sbi->s_next_generation++;
+	ext4_inode_set_gen(inode, sbi);
+	ext4_inode_set_gen(inode_bl, sbi);
 	spin_unlock(&sbi->s_next_gen_lock);
 
 	ext4_discard_preallocations(inode);
-- 
2.9.3