[PATCH -V5] ext4: Use -1 as the fake block number for delayed new buffer_head

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

ext4: Use a fake block number for delayed new buffer_head

From: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

Use a very large unsigned number (~0xffff) as as the fake block number
for the delayed new buffer. The VFS should never try to write out this
number, but if it does, this will make it obvious.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 fs/ext4/inode.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 498cf8b..4b1478c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2297,6 +2297,10 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 				  struct buffer_head *bh_result, int create)
 {
 	int ret = 0;
+	sector_t invalid_block = ~((sector_t) 0xffff);
+
+	if (invalid_block < ext4_blocks_count(EXT4_SB(inode->i_sb)->s_es))
+		invalid_block = ~0;
 
 	BUG_ON(create == 0);
 	BUG_ON(bh_result->b_size != inode->i_sb->s_blocksize);
@@ -2318,7 +2322,7 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 			/* not enough space to reserve */
 			return ret;
 
-		map_bh(bh_result, inode->i_sb, 0);
+		map_bh(bh_result, inode->i_sb, invalid_block);
 		set_buffer_new(bh_result);
 		set_buffer_delay(bh_result);
 	} else if (ret > 0) {